Train/Eval Mode Support

[ayeright]

Switch policies between train and eval mode.

Description

A recent PR in the stable-baselines3 library modified the policies so that they were switched between train and eval mode at the appropriate times. @araffin requested that this change also be implemented in this library. This PR makes this change for QR-DQN and TQC.

Context

• I have raised an issue to propose this change (required)

An issue was raised in the stable-baseline3 github.

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)

Checklist:

• I've read the CONTRIBUTION guide (required)
• The functionality/performance matches that of the source (required for new training algorithms or training-related features).
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have included an example of using the feature (required for new features).
• I have included baseline results (required for new training algorithms or training-related features).
• I have updated the documentation accordingly.
• I have updated the changelog accordingly (required).
• 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)

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

[araffin]

Thanks for the PR =)
I will wait for DLR-RM/stable-baselines3#558 to be merge so I can release a new version