Update beta sampling code in augment.py

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](https://colab.sandbox.google.com/gist/LakshmiKalaKadali/06533824610d6e85ea4aa3c6399819e6/tf_model_13490.ipynb#scrollTo=zSlE-3YDjL91) also attached. 

This PR closes [#13490](#13490)

Thank You

Author: LakshmiKalaKadali

Diff for: official/vision/ops/augment.py

@@ -2697,8 +2697,8 @@ def distort(self, images: tf.Tensor,
 
   @staticmethod
   def _sample_from_beta(alpha, beta, shape):
-    sample_alpha = tf.random.gamma(shape, 1., beta=alpha)
-    sample_beta = tf.random.gamma(shape, 1., beta=beta)
+    sample_alpha = tf.random.gamma(shape, alpha, beta=1.0)
+    sample_beta = tf.random.gamma(shape, alpha, beta=1.0)
     return sample_alpha / (sample_alpha + sample_beta)
 
   def _cutmix(self, images: tf.Tensor,