Add tensor support for num_spectrogram_bins in linear_to_mel_weight_matrix (#17404)

* Add tensor support for num_spectrogram_bins in linear_to_mel_weight_matrix

This fix tries to address the issue raised in 16553 where it was not possible
to provide num_spectrogram_bins as a tensor or placeholder for
linear_to_mel_weight_matrix.

The reason comes from the implementation of `_validate_arguments`
which requires num_spectrogram_bins to be a python.

However, the validation here is not necessary as `num_spectrogram_bins`
will be passed to `math_ops.linspace`, which performs the validation
anyway. The validation in `math_ops.linspace` is done in shape function
and in kernel's `Compute()`. For that it makes sense to remove the
validation of `num_spectrogram_bins` in `_validate_arguments` so that
the issue raised in 16553 could be addressed.
This fix adds a test case to cover the changes. Also, the
error case of `num_spectrogram_bins < 0` has already been covered in the
existing test case:
https://github.com/tensorflow/tensorflow/blob/013a6c7b3112573ba4d932c8a22bfaf45f648c77/tensorflow/contrib/signal/python/kernel_tests/mel_ops_test.py#L149-L165



This fix fixes 16553.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add test case for tensor support of num_spectrogram_bins in mel_ops.linear_to_mel_weight_matrix

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add comment for removing validation of num_spectrogram_bins

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update docstring

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update test case for num_spectrogram_bins

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Remove unused constant_op import to pass sanity check

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>