Repurpose torchdynamo training args towards torch._dynamo
# What does this PR do?

This PR re-uses the current `torchdynamo` training argument to be compatible with the internal module of PyTorch (in the nightlies). This is slightly breaking but at the same time the torchdynamo package has migrated to PyTorch proper, and the integration was marked as experimental.

The "fx2trt-fp16" backend is not advertised by PyTorch, so I removed it.
_The documentation is not available anymore as the PR was closed or merged._