Request for BFloat16 support in comms.py

[pruthvistony]

Currently train/comms/pt/comms.py supports only below data types.
['float32', 'int32', 'long', 'float16', 'float64', 'bool', 'Float', 'Int', 'Long', 'Double', 'Half', 'Bool', 'Byte']

cc @nrsatish