Allow Tensor lists to show up in symbolic differentiable graphs. (#16784)

Allow Tensor lists to show up in symbolic differentiable graphs. (#16784)

Summary:
It is done by flattening all tensor lists that are inputs/outputs to the
graph into the inputs/outputs list in the autograd graph.

This is less desirable than simply allowing IValues to exist in the
inputs/outputs of autograd::Function but it is substantially less
intrusive.

CaptureList describes the variables captured for backward in a single class.
UnpackInstructs describes how the flattened inputs to backwards are re-packed into lists.
ailzhang

This PR is also part 2 of covering maskrcnn & bert AD formulas, following #16689.

Ops added in this PR:
```
cat
index
meshgrid
reshape
split
split_with_sizes
stack
unbind
```
I will also add a few perf numbers here.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16784

Differential Revision: D14104063

Pulled By: ailzhang

fbshipit-source-id: 5ceadadfd67ccaac60c5fd6740786c5354e252b9