Summary:
Pull Request resolved: pytorch/pytorch#10824

API additions:
- Tensor(c10::intrusive_ptr<TensorImpl,UndefinedTensor>&&)
- Tensor(const c10::intrusive_ptr<TensorImpl,UndefinedTensor>&)
- Tensor::operator=(Tensor&&) && (for completeness sake)
- TensorBase::unsafeGetTensorImpl()
- TensorBase::unsafeReleaseTensorImpl()
- TensorBase::getIntrusivePtr()
- TensorImpl::type_id()
- Tensor::set_data()
- Tensor::is_same(Tensor)
- Tensor::use_count()
- Tensor::type_id()
- Tensor::scalar_type()
- WeakTensor::is_same(WeakTensor)
- intrusive_ptr::weak_use_count()
- weak_intrusive_ptr::weak_use_count()
- c10::raw::intrusive_ptr::{incref,decref,make_weak}
- c10::raw::weak_intrusive_ptr::{incref,decref,lock}

API changes:
- Tensor::pImpl is no longer public (and now named tensor_impl_)
    - Most methods accessed this way are now accessible on Tensor
      maybe_zero_dim() and set_wrapped_number() being prominent exceptions
      (they are now accessed through unsafeGetTensorImpl())
- Type is no longer friend of Tensor
- TensorBase::reset(TensorImpl*) is deleted
- TensorBase::reset(TensorImpl*, bool should_retain) is deleted
- TensorBase::swap(TensorBaseImpl&) is deleted; use std::swap instead
- TensorBase::get() is deleted; use unsafeGetTensorImpl() instead
- TensorBase::detach() is deleted; use unsafeReleaseTensorImpl() instead
- TensorBase::retain() is deleted; use _raw_incref() instead
- TensorBase::release() is deleted; use _raw_decref() instead
- WeakTensor lost most of its methods (it no longer inherits from
  TensorBase)
- TensorImpl::storage() is now a const method
- Tensor(TensorBase) constructor removed, instead
  we go through getIntrusivePtr().  I'm not sure about
  this change; I happened to have accidentally removed the
  TensorBase constructor and decided to fix call sites,
  but I could go the other way.
- detail::set_data() is deleted; use Tensor::set_data() instead
- c10::raw_intrusive_ptr_target removed; use the functions in c10::raw instead.
  (The reason for this change, is that it is invalid to cast an intrusive_ptr_target*
  to a raw_intrusive_ptr_target* to take advantage of the methods. But there is
  no reason the incref/decref methods shouldn't also work on intrusive_ptr_target;
  it is primarily an API consideration. We can be more standards compliant by
  keeping them as functions, which are universally applicable.)
- intrusive_ptr::reclaim() and weak_intrusive_ptr::reclaim() now work on
  pointers of the NullType. (This counts as a bug fix, because the documentation
  specified that pointers produced by release() are valid to reclaim(), and
  a release() on a null intrusive_ptr produces the NullType::singleton())

Bug fixes:
- Dispatch code for mutable references incorrectly returned
  a reference to a value argument (which would immediately
  go out of scope).  They now correctly return a tensor by
  value.
- intrusive_ptr copy/move assignment did not work correctly when
  an object was assigned to itself. We now check for this case and
  no-op if so. (This bug manifested itself as a Tensor mysteriously
  becoming an UndefinedTensor after lines of code like
  'x = x.mul_(y)')

Other changes:
- The checked cast functions in Utils.h have now been
  renamed and detemplatized into checked unwrap functions.
- Added type_id() and scalar_type() methods to Tensor
- pImpl is no longer public
- Documented what the && overloads are doing
- All occurrences of 'new TensorImpl' (and similar spellings, like 'new THTensor')
  have been expunged. This is NO LONGER a valid way to create a new
  tensor, and if you do this, upon your first incref, you will catch an ASSERT
  failure saying that only tensors created by intrusive_ptr::release() are valid
  to reclaim(). Use c10::make_intrusive instead in this situation.
- IValue is adjusted to use intrusive_ptr instead of Retainable, and all
  other sub-classes of Retainable were modified to use intrusive_ptr.
  When doing this, I had to make the constructors of sub-classes like
  ConstantList public, so that c10::make_intrusive could invoke them.  Fortunately,
  if you incorrectly stack allocate a ConstantList, and then try to get an
  intrusive_ptr to it, it will fail, as stack allocated ConstantLists have refcount 0.
- IValue very narrowly sidesteps the problem of handling NullType, as it
  considers intrusive_ptr<TensorImpl> identical to intrusive_ptr<TensorImpl, UndefinedTensor>
  which is not always true. This was always the case, but there's now a comment
  explaining what's going on.

Some MSVC bugs were uncovered during the preparation of this patch.
They are documented as comments in the code.

Reviewed By: gchanan

Differential Revision: D9481140

fbshipit-source-id: 14a8ea0c231ed88b5715fb86d92730926f9f92fc

Summary:
Pull Request resolved: pytorch#10824

API additions:
- Tensor(c10::intrusive_ptr<TensorImpl,UndefinedTensor>&&)
- Tensor(const c10::intrusive_ptr<TensorImpl,UndefinedTensor>&)
- Tensor::operator=(Tensor&&) && (for completeness sake)
- TensorBase::unsafeGetTensorImpl()
- TensorBase::unsafeReleaseTensorImpl()
- TensorBase::getIntrusivePtr()
- TensorImpl::type_id()
- Tensor::set_data()
- Tensor::is_same(Tensor)
- Tensor::use_count()
- Tensor::type_id()
- Tensor::scalar_type()
- WeakTensor::is_same(WeakTensor)
- intrusive_ptr::weak_use_count()
- weak_intrusive_ptr::weak_use_count()
- c10::raw::intrusive_ptr::{incref,decref,make_weak}
- c10::raw::weak_intrusive_ptr::{incref,decref,lock}

API changes:
- Tensor::pImpl is no longer public (and now named tensor_impl_)
    - Most methods accessed this way are now accessible on Tensor
      maybe_zero_dim() and set_wrapped_number() being prominent exceptions
      (they are now accessed through unsafeGetTensorImpl())
- Type is no longer friend of Tensor
- TensorBase::reset(TensorImpl*) is deleted
- TensorBase::reset(TensorImpl*, bool should_retain) is deleted
- TensorBase::swap(TensorBaseImpl&) is deleted; use std::swap instead
- TensorBase::get() is deleted; use unsafeGetTensorImpl() instead
- TensorBase::detach() is deleted; use unsafeReleaseTensorImpl() instead
- TensorBase::retain() is deleted; use _raw_incref() instead
- TensorBase::release() is deleted; use _raw_decref() instead
- WeakTensor lost most of its methods (it no longer inherits from
  TensorBase)
- TensorImpl::storage() is now a const method
- Tensor(TensorBase) constructor removed, instead
  we go through getIntrusivePtr().  I'm not sure about
  this change; I happened to have accidentally removed the
  TensorBase constructor and decided to fix call sites,
  but I could go the other way.
- detail::set_data() is deleted; use Tensor::set_data() instead
- c10::raw_intrusive_ptr_target removed; use the functions in c10::raw instead.
  (The reason for this change, is that it is invalid to cast an intrusive_ptr_target*
  to a raw_intrusive_ptr_target* to take advantage of the methods. But there is
  no reason the incref/decref methods shouldn't also work on intrusive_ptr_target;
  it is primarily an API consideration. We can be more standards compliant by
  keeping them as functions, which are universally applicable.)
- intrusive_ptr::reclaim() and weak_intrusive_ptr::reclaim() now work on
  pointers of the NullType. (This counts as a bug fix, because the documentation
  specified that pointers produced by release() are valid to reclaim(), and
  a release() on a null intrusive_ptr produces the NullType::singleton())

Bug fixes:
- Dispatch code for mutable references incorrectly returned
  a reference to a value argument (which would immediately
  go out of scope).  They now correctly return a tensor by
  value.
- intrusive_ptr copy/move assignment did not work correctly when
  an object was assigned to itself. We now check for this case and
  no-op if so. (This bug manifested itself as a Tensor mysteriously
  becoming an UndefinedTensor after lines of code like
  'x = x.mul_(y)')

Other changes:
- The checked cast functions in Utils.h have now been
  renamed and detemplatized into checked unwrap functions.
- Added type_id() and scalar_type() methods to Tensor
- pImpl is no longer public
- Documented what the && overloads are doing
- All occurrences of 'new TensorImpl' (and similar spellings, like 'new THTensor')
  have been expunged. This is NO LONGER a valid way to create a new
  tensor, and if you do this, upon your first incref, you will catch an ASSERT
  failure saying that only tensors created by intrusive_ptr::release() are valid
  to reclaim(). Use c10::make_intrusive instead in this situation.
- IValue is adjusted to use intrusive_ptr instead of Retainable, and all
  other sub-classes of Retainable were modified to use intrusive_ptr.
  When doing this, I had to make the constructors of sub-classes like
  ConstantList public, so that c10::make_intrusive could invoke them.  Fortunately,
  if you incorrectly stack allocate a ConstantList, and then try to get an
  intrusive_ptr to it, it will fail, as stack allocated ConstantLists have refcount 0.
- IValue very narrowly sidesteps the problem of handling NullType, as it
  considers intrusive_ptr<TensorImpl> identical to intrusive_ptr<TensorImpl, UndefinedTensor>
  which is not always true. This was always the case, but there's now a comment
  explaining what's going on.

Some MSVC bugs were uncovered during the preparation of this patch.
They are documented as comments in the code.

Reviewed By: gchanan

Differential Revision: D9481140

fbshipit-source-id: 14a8ea0c231ed88b5715fb86d92730926f9f92fc