Bump min PyTorch version

[araffin]

Description

Bump PyTorch version to be able to use the latest features (will be needed for Stable-Baselines-Team/stable-baselines3-contrib#62 for the covariance matrix), newest version also introduces bug fixes and performance improvements.
In practice, SB3 should still be usable with many PyTorch versions though.

Updated doc to link to Gym official doc website.

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

closes #813

Checklist:

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)
• I have checked that the documentation builds using make doc (required)

Note: You can run most of the checks using make commit-checks.

Note: we are using a maximum length of 127 characters per line

[Miffyli]

LGTM, agree with the point. Thanks for cleaning other URLs as well :). Thankfully PyTorch has been 95% backwards compatible in this regard.

My biggest worry is that saved models might not work as intended (or something small has changed there), but indeed, people can install other PyTorch version for this case.