Pass Variable into Caffe2 ops, by requiring that the Variable doesn't require grad (#22473)
Summary:
As part of the Variable/Tensor merge, we want to be able to pass Variables into Caffe2 without doing extra shallow copy, to improve performance and also allow for in-place mutations in Caffe2 ops. There are a few approaches outlined in https://github.com/pytorch/pytorch/pull/22418, and this PR is the chosen approach.
Specifically, we can have the assumption that we won't be connecting autograd to C2 gradients at any point (as it's too tricky and not that useful). Therefore, we can pass Variable into Caffe2 ops by requiring that all Variables in Caffe2 don't require grad. For code paths in Caffe2 that might potentially track gradients (e.g. `ScriptModuleOp` and `call_caffe2_op_from_c10`), we use the `torch::NoGradGuard` to make sure gradients are not tracked.
This supersedes https://github.com/pytorch/pytorch/pull/22418.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22473
Differential Revision: D16099042
Pulled By: yf225
fbshipit-source-id: 57efc3c7cfb3048d9abe90e63759acc14ebd2972
