Disable autocast cache for tensor views as fix for #48049 (#48696)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/48049
Root cause of the issue explained [here](https://github.com/pytorch/pytorch/issues/48049#issuecomment-736701769).
This PR implements albanD's suggestion to add the `!t.is_view()` check and disable autocast caching for views of tensors.
The added test checks for an increase in memory usage by comparing the initially allocated memory with the memory after 3 iterations using a single `nn.Linear` layer in a `no_grad` and `autocast` context.
After this PR the memory usage in the original issue doesn't grow anymore and yields:
```python
autocast: True
0: 0MB (peak 1165MB)
1: 0MB (peak 1264MB)
2: 0MB (peak 1265MB)
3: 0MB (peak 1265MB)
4: 0MB (peak 1265MB)
5: 0MB (peak 1265MB)
6: 0MB (peak 1265MB)
7: 0MB (peak 1265MB)
8: 0MB (peak 1265MB)
9: 0MB (peak 1265MB)
```
CC ngimel mcarilli
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48696
Reviewed By: bdhirsh
Differential Revision: D25276231
Pulled By: ngimel
fbshipit-source-id: e2571e9f166c0a6f6f569b0c28e8b9ca34132743
