Cleanup old batch sizes

[garrettwrong]

Update old (CPU only) batch sizes of 8192 with our preferred starting point of 512.

Adds a little blurb to cov2d docstring.

One of the action items from the cov2d comparison.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.84%. Comparing base (158e2a2) to head (ce211ac).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/aspire/commands/cov3d.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1365   +/-   ##
========================================
  Coverage    90.84%   90.84%           
========================================
  Files          135      135           
  Lines        14658    14658           
========================================
  Hits         13316    13316           
  Misses        1342     1342           

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[j-c-c]

LGTM!