Fix bug in Visualizer when plotting temporal data w/ batch size 1

[AdeelH]

Overview

Fixes #1956

Checklist

• Added needs-backport label if PR is bug fix that applies to previous minor release
• Ran scripts/format_code and committed any changes
• Documentation updated if needed
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

• See new unit tests.

[codecov]

Codecov Report

Merging #1958 (de92fc7) into master (e4e10ad) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1958   +/-   ##
=======================================
  Coverage   82.47%   82.47%           
=======================================
  Files         190      190           
  Lines        9337     9337           
=======================================
  Hits         7701     7701           
  Misses       1636     1636           

Files	Coverage Δ
...n/pytorch_learner/dataset/visualizer/visualizer.py	96.55% <100.00%> (ø)