kill _parameter_list (#27399)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27399
This was devised in a time when we didn't have module attributes. They
are essentially just tensor lists, so represent them that way. This has
the additional benefit of making the RNN forward pass faster because we
effectively cache the flattened weights.
The only complication part is that someone may come along and do:
```
my_rnn_mod.w_ih_l0 = torch.nn.Parameter(...)
```
This means we need to override setattr to keep the flattened weights
cache up to date.
Test Plan: Imported from OSS
Differential Revision: D17785658
Pulled By: suo
fbshipit-source-id: 7789cd1d0d4922bfd5eba1716976442fbf150766
