Add test for torchscripting nn.TransformerEncoder, including fast path (#79796)
Summary:
Add test just to check if TransformerEncoder will crash when enumerating over params [with_no_grad, use_torchscript, training].
Motivation for this was that TransformerEncoder fast path (so with_no_grad=True) and use_torchscript=True would crash with the issue that NestedTensor doesn't have size. This was caused because the TransformerEncoder fast path generates a NestedTensor automatically as a perf optimization and torchscript attempts to find intermediate tensor sizes while it optimizes. But NestedTensor has not implemented a size method, so things fail.
This test goes together with this fix https://github.com/pytorch/pytorch/pull/79480
Test Plan:
```
buck build --show-output mode/opt -c fbcode.enable_gpu_sections=true -c fbcode.nvcc_arch=a100 mode/inplace //caffe2/test:transformers
./fbcode/buck-out/gen/caffe2/test/transformers#binary.par
```
Test runs and passes together with the changes from the PR above (I made another diff on top of this with those changes). Does not pass without the fix.
Reviewed By: mikekgfb
Differential Revision: D37222923
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79796
Approved by: https://github.com/zrphercule
