Adding warnings in mu_law* for the wrong input type (#2034)

Summary:
Addresses  https://github.com/pytorch/audio/issues/1493

cc mthrok hwangjeff

Pull Request resolved: https://github.com/pytorch/audio/pull/2034

Reviewed By: hwangjeff

Differential Revision: D32807006

Pulled By: mthrok

fbshipit-source-id: badf148646c5f768328c5a4e51bd6016b0be46f3

torchaudio/functional/functional.py

@@ -612,7 +612,7 @@ def mu_law_encoding(
     r"""Encode signal based on mu-law companding.  For more info see the
     `Wikipedia Entry <https://en.wikipedia.org/wiki/%CE%9C-law_algorithm>`_
 
-    This algorithm assumes the signal has been scaled to between -1 and 1 and
+    This algorithm expects the signal has been scaled to between -1 and 1 and
     returns a signal encoded with values from 0 to quantization_channels - 1.
 
     Args:
@@ -624,6 +624,8 @@ def mu_law_encoding(
     """
     mu = quantization_channels - 1.0
     if not x.is_floating_point():
+        warnings.warn("The input Tensor must be of floating type. \
+            This will be an error in the v0.12 release.")
         x = x.to(torch.float)
     mu = torch.tensor(mu, dtype=x.dtype)
     x_mu = torch.sign(x) * torch.log1p(mu * torch.abs(x)) / torch.log1p(mu)