Require DOCTEST_SHOW environ to run plt.show (#96522)
@ezyang This is a minor change.
I was using the doctests to check that my install wasn't broken via:
```bash
xdoctest -m torch --style=google --global-exec "from torch import nn\nimport torch.nn.functional as F\nimport torch" --options="+IGNORE_WHITESPACE"
```
And noticed that it stops in the middle to show this matplotlib figure. I added a condition so it only does the pyplot show if a DOCTEST_SHOW environment variable exists. With this fix the above command runs to completion and is an easy way for users to put torch through its paces given just a fresh install.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96522
Approved by: https://github.com/ezyang
