Fix pip list format in collect_env (#16798)
Summary:
Since pip 18.0 (2018-07-22), `legacy` is no longer a valid choice for `pip list --format` as can be seen in the [Release Notes](https://pip.pypa.io/en/stable/news/#id62). Therefore, the options now are: `columns`, `freeze` and `json`. With `legacy`, this is how it looked like:
```
[...]
Versions of relevant libraries:
[pip3] numpy (1.16.1)
[pip3] torch (1.0.1)
[pip3] torchvision (0.2.1)
[...]
```
Changing to `freeze`, this is how it looks like:
```
[...]
Versions of relevant libraries:
[pip3] numpy==1.16.1
[pip3] torch==1.0.1
[pip3] torchvision==0.2.1
[...]
```
Currently, this is what happens:
```
[...]
Versions of relevant libraries:
[pip] Could not collect
[...]
```
The `freeze` option is also available in old pip, so this change is backwards compatible. Also, if we would like to keep the old style, which I think it is not necessary, I could easily change that.
---
In case anyone wants to know how `columns` looks like (I prefer `freeze`):
```
[...]
Versions of relevant libraries:
[pip3] numpy 1.16.1
[pip3] torch 1.0.1
[pip3] torchvision 0.2.1
[...]
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16798
Differential Revision: D13971793
Pulled By: soumith
fbshipit-source-id: 3721d9079a2afa245e1185f725598901185ea4cd
