implement sparsemax and sparsemax_loss

[AndreasMadsen]

• ENH: Including a Sparsemax Later and A SparsemaxLoss class

[seanpmorgan]

Generally looks great. Few things that need changes though

[AndreasMadsen]

Should be fixed.

[facaiy]

ignore it, my mistake.

[seanpmorgan]

Apologies for the fast merges in front of this issue. We had to finalize our styling etc.
@AndreasMadsen Could you please resolve conflicts and run make code-format

[AndreasMadsen]

Linting should be okay now.

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