Timer PR parts 1 and 2 from mosaicml#146 (mosaicml#174)

Added the `Time` class, `Timer` class, conversion function, and test cases and docstrings for these classes. This functionality encompasses parts 1 and 2 from mosaicml#146, with the following changes:

1. `tokens` will be 0 (instead of None) if not being tracked.

Also, modified the `StringEnum` class to relax the requirement that the lowercase name be the value, since this does not hold for our timing abbreviations.

Other PRs will integrate the Timer into the training loop.

composer/__init__.py

@@ -15,5 +15,8 @@
 from composer.core import Event as Event
 from composer.core import Logger as Logger
 from composer.core import State as State
+from composer.core import Time as Time
+from composer.core import Timer as Timer
+from composer.core import TimeUnit as TimeUnit
 from composer.core import types as types
 from composer.trainer import Trainer as Trainer

composer/core/__init__.py

@@ -8,3 +8,6 @@
 from composer.core.event import Event as Event
 from composer.core.logging import Logger as Logger
 from composer.core.state import State as State
+from composer.core.time import Time as Time
+from composer.core.time import Timer as Timer
+from composer.core.time import TimeUnit as TimeUnit