Add overdrive to functional

[bhargavkathivarapu]

Hi ,
Regarding the issue #260 ( Reducing dependency in Sox)
Implemented overdrive function in functional.py

• function.rst - updated with new fucntion

• test_sox_compatibility.py - Added a test for overdrive . Tests passed

...................gain: can't reclaim headroom
.
----------------------------------------------------------------------
Ran 20 tests in 509.658s

OK

• test_torchscript_consistency.py - added test . Test passed

...................................sssssssssssssssssssssssssssssssssss................ssssssssssssssss
----------------------------------------------------------------------
Ran 102 tests in 1400.141s

OK (skipped=51)

• test_batch_consistency.py - added test for overdrive . Test for overdrive passed, but
  existing batch_mfcc failed

...............F..
======================================================================
FAIL: test_batch_mfcc (__main__.TestTransforms)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/test_batch_consistency.py", line 201, in test_batch_mfcc
    torch.testing.assert_allclose(computed, expected, atol=1e-5, rtol=1e-5)
  File "/Users/ka387861/pytorch/torch/testing/__init__.py", line 59, in assert_allclose
    count - 1, 100 * count / actual.numel()))
AssertionError: Not within tolerance rtol=1e-05 atol=1e-05 at input[2, 0, 2, 1384] (0.5488258004188538 vs. 0.5488020181655884) and 8 other locations (0.00%)

----------------------------------------------------------------------
Ran 18 tests in 185.586s

For making the torch script test work i added a redundant operation .
Comment mentioning that is present in code.

@vincentqb could you please review these changes

[mthrok]

Thanks for PR. Mostly looks good!.

[mthrok]

Looks good to me. Thanks!