Make transformers as a optional requirement

[ntkathole]

Issue

Some users uses feast purely for feature management. Having pytorch/transformers in default installation increases the import time of the library.

Related to feast-dev/feast-workshop#23

Steps to reproduce

from time import perf_counter
now=perf_counter()
import feast
print(perf_counter()-now)

Possible Solution

Make transformers as a optional requirement in feast.