Added a third layer initialization option of kaiming he

Description

Add a third option for weight_init as 'kaiming he' (see https://pytorch.org/docs/stable/nn.init.html and https://arxiv.org/pdf/1803.08494).

Motivation and Context

This change adds additional option for layer initialization, providing more options for the user. In my personal work, I have found this to perform better than orthogonal.

closes #51

• 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)

Checklist

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (required).
  • I am unsure if I should actually change the changelogs given how sparse they are.
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
  • There are currently no tests in place for layer initialization.
• I have updated the documentation accordingly.
• I have opened an associated PR on the rllte-hub repository (if necessary)
• I have reformatted the code using make format (required)
  • make format formats the code but none of the formats are related to my changes.
• 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)

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