…e2_core_hip

* 'caffe2_core_hip' of github.com:petrex/pytorch: (24 commits)
  Allow empty storage for the 'Edge' class. (pytorch#7595)
  Process group base class and Gloo implementation (pytorch#7628)
  _LRSchedulers getstate include optimizer info (pytorch#7757)
  [PyTorch] [gradcheck] change backward() to grad() (pytorch#7710)
  Update test_nn.py (pytorch#7787)
  Define general default scheduler for TBB and fix ppc64le bug (pytorch#7761)
  Add support for accepting Tensor as input in clip_grad_*  functions. (pytorch#7769)
  [Easy] Remove unused code (pytorch#7782)
  Update tbb (pytorch#7734)
  Add @generated annotation (pytorch#7780)
  fix legacy comment after variable tensor merge (pytorch#7771)
  Revert pytorch#7750 and pytorch#7762 to fix Windows CI on master (pytorch#7772)
  Temporarily disable build env check (pytorch#7768)
  Add missing brace (pytorch#7762)
  [C++ API] Add backward() to Tensor and Variable  (pytorch#7750)
  [auto] Update onnx to d43b550 - Fix .gitignore and add missing files (onnx/onnx#1005) onnx/onnx@d43b550
  [auto] Update onnx to ea1aa13 - add tests for reduce ops (onnx/onnx#675) onnx/onnx@ea1aa13
  include cudnn_h (pytorch#7749)
  [C++ API] Using new registration mechanism (pytorch#7663)
  [auto] Update onnx to 5dd68e6 - Add a util function: polish_model (onnx/onnx#1000) onnx/onnx@5dd68e6
  ...

This commit:
- Converts edge storage to an optional type.
- Adds a new test in tarjans_test.
- Refactors related bits in other files.