Sparsemax layer does not actually support marking even though the support_masking flag is True

[pushpendre]

Describe the bug

The Sparsemax layer has the supports_masking flag True but its call method does not actually take in a mask.

Code to reproduce the issue

See https://github.com/tensorflow/addons/blob/v0.17.0/tensorflow_addons/layers/sparsemax.py#L36

Other info / logs

This issue can be fixed by supporting the use of mask

BIG_NUMBER = 1e6
def call(self, inputs, mask=None):
        return sparsemax(inputs - (1 - mask) * BIG_NUMBER, axis=self.axis) * mask

This code can be tested that it works as follows

    import numpy as np
    from tensorflow_addons.activations.tests.sparsemax_test import _np_sparsemax

    att_scores = np.random.randn(2, 3, 5, 7).astype(np.float32)
    mask = (np.random.uniform(0, 1, (2, 1, 5, 7)) > 0.5).astype(np.float32)
    
    out_sparsemax = Sparsemax()(att_scores, mask).numpy()
    for i in range(2):
        for j in range(3):
            for k in range(5):
                m = mask[i,0,k].astype(bool)
                a = out_sparsemax[i,j,k][m]
                b = _np_sparsemax(att_scores[i,j,k][m][np.newaxis, :])[0]
                assert np.allclose(a, b, atol=1e-6)

[bhack]

Can you extend the current test with a PR and then commit a fix?

[pushpendre]

i have a couple of deadlines coming up, I'll try to put something together probably after Sep 15.