Made a += b for lists do an in place add (#21896)
Summary:
In talks with smessmer, we decided that it'd be better to put the logic in `list`, as optimal behavior requires knowing `.capacity()`
Results on my cpu (for the benchmark here: https://twitter.com/VahidK/status/1138674536679821312) now look like this:
```
Pytorch batch_gather took 0.018311 seconds.
Pytorch batch_gather jit took 0.013921 seconds.
Pytorch vectorized batch_gather took 0.001384 seconds.
```
Previously, `batch_gather jit` took 3x as long as `batch_gather`.
Some logic taken from https://github.com/pytorch/pytorch/pull/21690. Note that these two PR's are somewhat orthogonal. That PR handles this benchmark by looking at the alias analysis, while this PR specializes for `+=`.
Note that we can't jit the vectorized version as we think `torch.arange` returns a float tensor.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21896
Differential Revision: D15998628
Pulled By: Chillee
fbshipit-source-id: b0085960da4613578b94deb98ac62c0a4532a8c3
