Update import paths to reflect fastai2->fastai rename

[iancoffey]

Description

Going through the code comments made it pretty confusing since the import paths across projects vary.

This just updates those comments with what works.

Motivation and Context

How Has This Been Tested?

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature and improvements (non-breaking change which adds/improves functionality)
• Bug fix (non-breaking change which fixes an issue)
• Code Refactoring (internal change which is not user facing)
• Documentation
• Test, CI, or build

Checklist:

• My code follows the bentoml code style, both ./dev/format.sh and
  ./dev/lint.sh script have passed
  (instructions).
• My change reduces project test coverage and requires unit tests to be added
• I have added unit tests covering my code change
• My change requires a change to the documentation
• I have updated the documentation accordingly

[codecov]

Codecov Report

Merging #1227 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1227   +/-   ##
=======================================
  Coverage   65.98%   65.98%           
=======================================
  Files         135      135           
  Lines        8563     8563           
=======================================
  Hits         5650     5650           
  Misses       2913     2913           

Impacted Files	Coverage Δ
bentoml/frameworks/fastai.py	0.00% <ø> (ø)

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[parano]

Thank you for fixing this @iancoffey !