Add mmcr projection head

[LukeSutor]

Add MMCR Projection Head as proposed in #1481

[LukeSutor]

@guarin When implementing the use_bias functionality for the ProjectionHead class, won't it now be necessary to change all of the types for the lists inside of the other ProjectionHead classes to include the union of the two types? (e.g. https://github.com/lightly-ai/lightly/blob/e84f85896bcaedeb1ff63b9c4bcf7c9d2885bcdf/lightly/models/modules/heads.py#L162).

[guarin]

@guarin When implementing the use_bias functionality for the ProjectionHead class, won't it now be necessary to change all of the types for the lists inside of the other ProjectionHead classes to include the union of the two types? (e.g.

I don't think it is necessary if the ProjectionHead.__init__ method has the following type:

    def __init__(
        self, 
        blocks: List[
            Union[
                Tuple[int, int, Optional[nn.Module], Optional[nn.Module]], # w/o use_bias term
                Tuple[int, int, Optional[nn.Module], Optional[nn.Module], bool], # with use_bias term
            ]
        ],

This will allow it to accept either the old lists with 4 entries or the new ones with 5 entries.

---

And could you add the class to this list in the tests: https://github.com/lightly-ai/lightly/blob/e84f85896bcaedeb1ff63b9c4bcf7c9d2885bcdf/tests/models/test_ProjectionHeads.py#L36-L51

Then it is also tested and we know it works as expected.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (715844b) 84.40% compared to head (7322153) 84.43%. Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1493 +/- ## ========================================== + Coverage 84.40% 84.43% +0.02% ========================================== Files 140 140 Lines 5783 5794 +11 ========================================== + Hits 4881 4892 +11 Misses 902 902 ```

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[guarin]

Thanks for the update! The code looks good but it seems that there is now a duplicate MMCRProjectionHead in the file.