CPU/GPU memory benchmarking utilities - Remove support for python 3.5 (now only 3.6+) (#3186)

* memory benchmark rss

* have both forward pass and line-by-line mem tracing

* cleaned up tracing

* refactored and cleaning up API

* no f-strings yet...

* add GPU mem logging

* fix GPU memory monitoring

* style and quality

* clean up and doc

* update with comments

* Switching to python 3.6+

* fix quality

docs/source/installation.md

@@ -1,6 +1,6 @@
 # Installation
 
-Transformers is tested on Python 3.5+ and PyTorch 1.1.0
+Transformers is tested on Python 3.6+ and PyTorch 1.1.0
 
 ## With pip