Add more timers

[anishbh]

This PR adds more timers to axonn.

Timers have been added to the forward/backward passes, the matrix multiplies, all-reduce, drop, reduce-scatter, and gather.

[siddharth9820]

Can you fix the formatting?
Install the black and flake8 packages using pip (pip install flake8 black)

To run black - black ./
To run flake8 - flake8
black automatically formats your code. flake8 only generates errors and you have to manually fix the formatting.

[siddharth9820]

lgtm, just needs formatting fixes.