Add PixelUnshuffle (#49334)
Summary:
Adds an implementation of `torch.nn.PixelUnshuffle` as the inverse operation of `torch.nn.PixelShuffle`. This addresses https://github.com/pytorch/pytorch/issues/2456
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49334
Test Plan:
```
# Unit tests.
python test/test_nn.py TestNN.test_pixel_shuffle_unshuffle
# Module test.
python test/test_nn.py TestNN.test_PixelUnshuffle
# C++ API tests.
build/bin/test_api
# C++ / python parity tests.
python test/test_cpp_api_parity.py
# JIT test.
python test/test_jit.py TestJitGeneratedFunctional.test_nn_pixel_unshuffle
# Override tests.
python test/test_overrides.py
# Type hint tests.
python test/test_type_hints.py
```
Screenshots of rendered docs:
<img width="876" alt="Screen Shot 2020-12-18 at 12 19 05 PM" src="https://user-images.githubusercontent.com/75754324/102642255-6b07bb00-412b-11eb-88fa-e53e7e8ba720.png">
<img width="984" alt="Screen Shot 2020-12-18 at 12 19 26 PM" src="https://user-images.githubusercontent.com/75754324/102642276-70fd9c00-412b-11eb-8548-445082a2db02.png">
<img width="932" alt="Screen Shot 2020-12-18 at 12 19 34 PM" src="https://user-images.githubusercontent.com/75754324/102642704-19abfb80-412c-11eb-9546-95bdd1c3cf22.png">
<img width="876" alt="Screen Shot 2020-12-22 at 12 51 36 PM" src="https://user-images.githubusercontent.com/75754324/102918259-986aa680-4454-11eb-99e7-a0b4c8b3e283.png">
<img width="869" alt="Screen Shot 2020-12-22 at 12 51 44 PM" src="https://user-images.githubusercontent.com/75754324/102918274-9ef91e00-4454-11eb-94bb-91b58aff47d3.png">
Reviewed By: mruberry
Differential Revision: D25401439
Pulled By: jbschlosser
fbshipit-source-id: 209d92ce7295e51699e83616d0c62170a7ce75c8
