Add lazy loading to `add_config`

[PetrochukM]

At the moment, add_config imports every module that is referenced in the configuration in order to check the function signature and path. With this PR, we add an option to lazily import modules only if they are already in sys.modules.

This optionality should increase initialization speed but it potentially leaves room for some configurations to not be checked because they are never run during runtime.

Finally:

• Added a couple of comments
• Added a conftest file so that pytest rests the global state before each test.
• Remove redundant call to inspect.stack
• Improve HParam initialization performance like so: https://stackoverflow.com/questions/17407119/python-inspect-stack-is-slow

[codecov-io]

Codecov Report

Merging #3 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #3   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines         226    247   +21     
=====================================
+ Hits          226    247   +21

Impacted Files	Coverage Δ
hparams/__init__.py	100% <100%> (ø)	:arrow_up:
hparams/hparams.py	100% <100%> (ø)	:arrow_up:

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