Fixing unittest + mkdocs errors

[DomInvivo]

Changelogs

• Fixed the masking in the Attention by simply casting to bool. The mask was changed to float somewhere in TransformerEncoder call to F._canonical_mask.
• Fixed the double VS float of the test_positional_encoders.py by simply casting the positional encoding to float. Casting is usually handled in the dataloading anyways.
• Fixed the IPU tests by moving the IPU imports within the @pytest.mark.ipu decorator
• Fixed the mkdocs documentation building by removing broken references
• Upgraded the python version for the unit-test to 3.11, and removed 3.08

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Simply fixing minor issues with unittests and mkdocs