Rename batch sizes to give informative units

[tomMcGrath]

Description

Previously 'batch size' could refer to number of input sequences (for the activations store) or total number of tokens (i.e. number of sequences * sequence length). This inconsistency made these numbers look confusing when compared. This PR renames these values to make the units clearer.

Fixes #128

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility (except to rename the relevant variables!)

You have tested formatting, typing and unit tests (acceptance tests not currently in use)

• I have run make check-ci to check format and linting. (you can run make format to format code if needed.)

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 64.47%. Comparing base (007141e) to head (76a9b7e).

Files	Patch %	Lines
sae_lens/training/evals.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #133   +/-   ##
=======================================
  Coverage   64.47%   64.47%           
=======================================
  Files          17       17           
  Lines        1782     1782           
  Branches      296      296           
=======================================
  Hits         1149     1149           
  Misses        567      567           
  Partials       66       66           

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