Hi!

I was playing with libfuzzer and found bug when loading a model from file via `torch::jit::load` function.
There is an unhandled exception in caffe2/serialize when calling a `stoull` function on unsanitized version string.

The bug can be reproduced with `aot_model_compiler` binary:
```
aot_model_compiler --model=crash-stoull --model_name=name --model_version=1 --input_dims='1,3,224,224;2,2' --input_types='float;float'
```

Crash file is provided in [crash.zip](https://github.com/pytorch/pytorch/files/8701504/crash.zip).

gdb output:
```
Temporary breakpoint 1, main (argc=6, argv=0x7ffcd160f9f8) at /pytorch_master/binaries/aot_model_compiler.cc:87
87	      "Run NNC AOT compiler for pytorch model. Example usage:\n"
(gdb) c
Continuing.
terminate called after throwing an instance of 'std::invalid_argument'
  what():  stoull

Program received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50	../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007fa637f16859 in __GI_abort () at abort.c:79
pytorch#2  0x00007fa6381c1911 in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
pytorch#3  0x00007fa6381cd38c in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
pytorch#4  0x00007fa6381cd3f7 in std::terminate() () from /lib/x86_64-linux-gnu/libstdc++.so.6
pytorch#5  0x00007fa6381cd6a9 in __cxa_throw () from /lib/x86_64-linux-gnu/libstdc++.so.6
pytorch#6  0x00007fa6381c42ce in std::__throw_invalid_argument(char const*) () from /lib/x86_64-linux-gnu/libstdc++.so.6
pytorch#7  0x000000000247d567 in __gnu_cxx::__stoa<unsigned long long, unsigned long long, char, int> (__str=0x7ffcd160f228 "ZZ", __idx=0x0, __base=10, __convf=<optimized out>, __name=<optimized out>)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/ext/string_conversions.h:83
pytorch#8  std::__cxx11::stoull (__str="ZZ", __idx=0x0, __base=10) at /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/basic_string.h:6577
pytorch#9  caffe2::serialize::PyTorchStreamReader::init (this=this@entry=0x8c11ce0) at /pytorch_master/caffe2/serialize/inline_container.cc:145
pytorch#10 0x000000000247d9c7 in caffe2::serialize::PyTorchStreamReader::PyTorchStreamReader (this=0x8c11ce0, in=std::shared_ptr<class caffe2::serialize::ReadAdapterInterface> (empty) = {...})
    at /pytorch_master/caffe2/serialize/inline_container.cc:88
pytorch#11 0x00000000035b7ba4 in __gnu_cxx::new_allocator<caffe2::serialize::PyTorchStreamReader>::construct<caffe2::serialize::PyTorchStreamReader, std::shared_ptr<caffe2::serialize::ReadAdapterInterface> > (
    __p=0x2, __args=..., this=<optimized out>) at /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/ext/new_allocator.h:150
pytorch#12 std::allocator_traits<std::allocator<caffe2::serialize::PyTorchStreamReader> >::construct<caffe2::serialize::PyTorchStreamReader, std::shared_ptr<caffe2::serialize::ReadAdapterInterface> > (__a=...,
    __p=0x2, __p@entry=0x8c11ce0, __args=...) at /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/alloc_traits.h:512
pytorch#13 0x00000000035b1988 in std::_Sp_counted_ptr_inplace<caffe2::serialize::PyTorchStreamReader, std::allocator<caffe2::serialize::PyTorchStreamReader>, (__gnu_cxx::_Lock_policy)2>::_Sp_counted_ptr_inplace<std::shared_ptr<caffe2::serialize::ReadAdapterInterface> > (this=0x8c11cd0, __a=..., __args=...) at /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/shared_ptr_base.h:551
pytorch#14 std::__shared_count<(__gnu_cxx::_Lock_policy)2>::__shared_count<caffe2::serialize::PyTorchStreamReader, std::allocator<caffe2::serialize::PyTorchStreamReader>, std::shared_ptr<caffe2::serialize::ReadAdapterInterface> > (this=0x7ffcd160f3a8, __p=@0x7ffcd160f3a0: 0x10, __args=..., __a=...) at /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/shared_ptr_base.h:683
pytorch#15 std::__shared_ptr<caffe2::serialize::PyTorchStreamReader, (__gnu_cxx::_Lock_policy)2>::__shared_ptr<std::allocator<caffe2::serialize::PyTorchStreamReader>, std::shared_ptr<caffe2::serialize::ReadAdapterInterface> > (this=0x7ffcd160f3a0, __args=..., __tag=...) at /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/shared_ptr_base.h:1371
pytorch#16 std::shared_ptr<caffe2::serialize::PyTorchStreamReader>::shared_ptr<std::allocator<caffe2::serialize::PyTorchStreamReader>, std::shared_ptr<caffe2::serialize::ReadAdapterInterface> > (this=0x7ffcd160f3a0,
    __args=..., __tag=...) at /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/shared_ptr.h:408
pytorch#17 std::allocate_shared<caffe2::serialize::PyTorchStreamReader, std::allocator<caffe2::serialize::PyTorchStreamReader>, std::shared_ptr<caffe2::serialize::ReadAdapterInterface> > (__args=..., __a=...)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/shared_ptr.h:859
pytorch#18 std::make_shared<caffe2::serialize::PyTorchStreamReader, std::shared_ptr<caffe2::serialize::ReadAdapterInterface> > (__args=...)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/shared_ptr.h:875
pytorch#19 torch::jit::load (rai=std::shared_ptr<class caffe2::serialize::ReadAdapterInterface> (empty) = {...}, device=device@entry=..., Python Exception <class 'gdb.error'> No type named std::__detail::_Hash_node<struct std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, true>.:
extra_files=std::unordered_map with 0 elements)
    at /pytorch_master/torch/csrc/jit/serialization/import.cpp:474
pytorch#20 0x00000000035b1ef6 in torch::jit::load (filename="crash-stoull", device=device@entry=..., Python Exception <class 'gdb.error'> No type named std::__detail::_Hash_node<struct std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, true>.:
extra_files=std::unordered_map with 0 elements) at /pytorch_master/torch/csrc/jit/serialization/import.cpp:444
pytorch#21 0x00000000035b1d22 in torch::jit::load (filename="", device=device@entry=...) at /pytorch_master/torch/csrc/jit/serialization/import.cpp:424
pytorch#22 0x00000000008f9be3 in main (argc=1, argv=0x7ffcd160f9f8) at /pytorch_master/binaries/aot_model_compiler.cc:128
```

Pull Request resolved: pytorch#77557
Approved by: https://github.com/Gamrix

### Summary:
This PR implements QAT for APoT FakeQuant. It runs QAT with FX graph mode quantized models (Resnet-18 pre-trained model, full ImageNet dataset) to compare accuracy metrics for different qconfig settings of uniform vs. APoT quantized activation and weight. It also refactors the APoT PTQ module `apot_fx_graph_mode_ptq.py` (previously `fx_graph_mode_apot.py`) such that shared helper functions between PTQ and QAT are in a separate file `quantization_util.py`.

Model pytorch#2 (uniformly quantized activation, APoT quantized weight) shows comparable accuracy compared to model #1 (uniformly quantized activation, APoT quantized weight) for 8-bit and significant accuracy improvement for 4-bit (see "Accuracy Stats" section below).

### Test Plan:
Run QAT models with: `python test/quantization/core/experimental/apot_qat.py`
Run PTQ models with: `python test/quantization/core/experimental/apot_ptq.py`

### Accuracy Stats
8-bit (Uniform int8, APoT b = 8 k = 2)

Model #1: Uniform activation, uniform weight (FX Graph Mode quantized)
Evaluation accuracy on test dataset: 69.67% (Top-1), 89.04% (Top-5)

Model pytorch#2: Uniform activation, APoT weight (FX Graph Mode quantized)
Evaluation accuracy on test dataset: 69.72% (Top-1), 89.06% (Top-5)

4-bit (Uniform int4, APoT b = 4 k = 2)

Model #1: Uniform activation, uniform weight (FX Graph Mode quantized)
Evaluation accuracy on test dataset: 46.85% (Top-1), 72.85% (Top-5)

Model pytorch#2: Uniform activation, APoT weight (FX Graph Mode quantized)
Evaluation accuracy on test dataset: 66.45% (Top-1), 86.23% (Top-5)
Pull Request resolved: pytorch#83282
Approved by: https://github.com/jerryzh168