fix: Fixed LR Finder plot scaling

[frgfm]

This PR fixes the plot limits for LR Find. The numpy .argmax method yields a numpy.int64 which does not verify the check isinstance(min_idx, int) so the scaling was off in most cases.

[codecov]

Codecov Report

Merging #147 (b6f6678) into master (db8c5fe) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   91.98%   91.98%           
=======================================
  Files          52       52           
  Lines        3007     3007           
=======================================
  Hits         2766     2766           
  Misses        241      241           

Impacted Files	Coverage Δ
holocron/trainer/core.py	88.29% <100.00%> (ø)