[fx2trt] Add layernorm plugin for dynamic shape (#63620)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63620
Added layernorm dynamic plugin, so that it works when explicit batch dim is required. Needed for ig model.
Changed the way of how we creating a plugin layer from instantiating the plugin directly to use plugin creator with `PluginFieldCollection`.
Follow ups:
Another way to convert layernorm is by breaking it down to supported trt layers. T97398182
Test Plan: layernorm unittest
Reviewed By: yinghai
Differential Revision: D30138205
fbshipit-source-id: aebe021d8de818e20376634f30e84579b9807f9b
