Autocast support for cudnn RNNs (#42385)
Summary:
Should close https://github.com/pytorch/pytorch/issues/36428.
The cudnn RNN API expects weights to occupy a flat buffer in memory with a particular layout. This PR implements a "speed of light" fix: [`_cudnn_rnn_cast_reflatten`](https://github.com/pytorch/pytorch/pull/42385/files#diff-9ef93b6a4fb5a06a37c562b83737ac6aR327) (the autocast wrapper assigned to `_cudnn_rnn`) copies weights to the right slices of a flat FP16 buffer with a single read/write per weight (as opposed to casting them to FP16 individually then reflattening the individual FP16 weights, which would require 2 read/writes per weight).
It isn't pretty but IMO it doesn't make rnn bindings much more tortuous than they already are.
The [test](https://github.com/pytorch/pytorch/pull/42385/files#diff-e68a7bc6ba14f212e5e7eb3727394b40R2683) tries a forward under autocast and a backward for the full cross product of RNN options and input/weight/hidden dtypes. As for all FP16list autocast tests, forward output and backward grads are checked against a control where inputs (including RNN module weights in this case) are precasted to FP16 on the python side.
Not sure who to ask for review, tagging ezyang and ngimel because Ed wrote this file (almost 2 years ago) and Natalia did the most recent major [surgery](https://github.com/pytorch/pytorch/pull/12600).
Side quests discovered:
- Should we update [persistent RNN heuristics](https://github.com/pytorch/pytorch/blob/dbdd28207c5cf6c4a35ceb1de0811c4812e8882c/aten/src/ATen/native/cudnn/RNN.cpp#L584) to include compute capability 8.0? Could be another PR but seems easy enough to include.
- Many (maybe all?!) the raw cudnn API calls in [RNN.cpp](https://github.com/pytorch/pytorch/blob/master/aten/src/ATen/native/cudnn/RNN.cpp) are deprecated in cudnn 8. I don't mind taking the AI to update them since my mental cache is full of rnn stuff, but that would be a substantial separate PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42385
Reviewed By: zhangguanheng66
Differential Revision: D23077782
Pulled By: ezyang
fbshipit-source-id: a2afb1bdab33ba0442879a703df13dc87f03ec2e
