Validate argument minvalue of tf.random.uniform
Hi @cantonios Can you please review this PR ? Thank you!
At present the API, `tf.random.uniform` raises `InvalidArgumentError` if `minval` >= `maxval` for `int` dtype only. This check happens in C++ backend for `int` data type only.

However same is not true for `float` datatype. It will not raise an error and It is ignoring min and max values in this case considering the range only.

The behaviour should be same for `int` and `float` dtypes. Hence for uniformity, I am adding validation for same at python level itself.

Attaching gist for same.

This is effecting `keras.initializers.RandomUniform` also which was discussed in tf-kears repo at #[449](https://github.com/keras-team/tf-keras/issues/449)