chore: Updated license from MIT to Apache2.0

[frgfm]

This PR simply updates the license from MIT to Apache 2.0

[codecov]

Codecov Report

Merging #130 (6d63af1) into master (5234f4f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #130   +/-   ##
=======================================
  Coverage   91.03%   91.03%           
=======================================
  Files          51       51           
  Lines        2935     2935           
=======================================
  Hits         2672     2672           
  Misses        263      263           

Impacted Files	Coverage Δ
holocron/models/darknet.py	100.00% <ø> (ø)
holocron/models/darknetv2.py	100.00% <ø> (ø)
holocron/models/darknetv3.py	100.00% <ø> (ø)
holocron/models/darknetv4.py	100.00% <ø> (ø)
holocron/models/detection/yolo.py	98.03% <ø> (ø)
holocron/models/detection/yolov2.py	98.85% <ø> (ø)
holocron/models/detection/yolov4.py	98.03% <ø> (ø)
holocron/models/pyconvresnet.py	100.00% <ø> (ø)
holocron/models/repvgg.py	89.15% <ø> (ø)
holocron/models/res2net.py	95.83% <ø> (+0.08%)	⬆️
... and 30 more