Update beta sampling code in augment.py

[LakshmiKalaKadali]

The function _sample_from_beta(alpha, beta, shape) in MixupAndCutmix class, is not having the same functionality as numpy.random.beta. So tfm.vision.augment.MixupAndCutmix._sample_from_beta(0.2, 0.2, tf.shape( tf.range(10000))).numpy() is also deviating as well. So suggesting the fix keeping alpha=alpha, beta=1.0 in _sample_from_beta. The reproduced gist also attached.

This PR closes #13490

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