[fx2trt] Add support for explicit batch dimension (#62110)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62110
Add an option to opt in explicit batch dimension. Extend unit tests to test both scenario (implicit and explicit). Fixed some converters that doesn't work with explicit batch dimension before.
Add broadcast support and a generic function for adding elementwise binary ops.
Follow ups:
1. Adding the dynamic shape support in explicit batch dimension mode to allow different batch dimension at least.
2. Extend layer_norm plugin `PluginV2Ext` to make it work in explicit batch dimension.
Test Plan: unit tests
Reviewed By: jackm321
Differential Revision: D29798239
fbshipit-source-id: 91d47c6155d2473ed4a6f8d2816715a32c61b869
