[converters/stack.py] Update stack converter to support dynamic shapes. Resolves #830.

[chaoz-dev]

Resolves issue #830, where torch.stack does not currently correctly handle dynamic shapes during TRT conversion.

Checked with included test:

python torch2trt/converters/stack_test.py

[jaybdub]

Thanks for the PR!

I believe when I was working on dynamic shapes for either stack or chunk, I had an issue where adding dynamic shape support broke static models.

It's been a while so I forget what the concern was exactly, but I may want to sanity check a few things first.