Huber loss (#50553)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/48595.
## Background
This PR implements HuberLoss, which differs from SmoothL1Loss by a factor of beta. The current implementation does not share logic between the two. Feedback is welcome for the optimal way to minimize code duplication while remaining performant.
I've done some early [benchmarking](https://pytorch.org/tutorials/recipes/recipes/benchmark.html#collecting-instruction-counts-with-callgrind) with Huber calling in to the Smooth L1 kernel and scaling afterwards; for the simple test case I used, instruction counts are as follows:
```
Huber loss calls dedicated Huber kernel: 2,795,300
Huber loss calls Smooth L1 kernel and scales afterwards: 4,523,612
```
With these numbers, instruction counts are ~62% higher when using the pre-existing Smooth L1 kernel.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50553
Test Plan:
```
python test/test_nn.py TestNN.test_HuberLoss
python test/test_nn.py TestNN.test_HuberLoss_delta
python test/test_nn.py TestNN.test_huber_loss_invalid_delta
python test/test_nn.py TestNNDeviceTypeCPU.test_smooth_l1_loss_vs_huber_loss_cpu
python test/test_nn.py TestNNDeviceTypeCUDA.test_smooth_l1_loss_vs_huber_loss_cuda
python test/test_nn.py TestNNDeviceTypeCPU.test_invalid_reduction_strings_cpu
python test/test_nn.py TestNNDeviceTypeCUDA.test_invalid_reduction_strings_cuda
python test/test_nn.py TestNN.test_loss_equal_input_target_shape
python test/test_nn.py TestNN.test_pointwise_loss_broadcast
python test/test_overrides.py
python test/test_jit.py TestJitGeneratedFunctional.test_nn_huber_loss
python test/test_type_hints.py
python test/test_cpp_api_parity.py
build/bin/test_api
```
## Documentation
<img width="677" alt="Screen Shot 2021-01-14 at 4 25 08 PM" src="https://user-images.githubusercontent.com/75754324/104651224-5a445980-5685-11eb-884b-14ea517958c2.png">
<img width="677" alt="Screen Shot 2021-01-14 at 4 24 35 PM" src="https://user-images.githubusercontent.com/75754324/104651190-4e589780-5685-11eb-974d-8c63a89c050e.png">
<img width="661" alt="Screen Shot 2021-01-14 at 4 24 45 PM" src="https://user-images.githubusercontent.com/75754324/104651198-50225b00-5685-11eb-958e-136b36f6f8a8.png">
<img width="869" alt="Screen Shot 2021-01-14 at 4 25 27 PM" src="https://user-images.githubusercontent.com/75754324/104651208-53b5e200-5685-11eb-9fe4-5ff433aa13c5.png">
<img width="862" alt="Screen Shot 2021-01-14 at 4 25 48 PM" src="https://user-images.githubusercontent.com/75754324/104651209-53b5e200-5685-11eb-8051-b0cfddcb07d3.png">
Reviewed By: H-Huang
Differential Revision: D26734071
Pulled By: jbschlosser
fbshipit-source-id: c98c1b5f32a16f7a2a4e04bdce678080eceed5d5
