fix bug in image inference

[themattinthehatt]

When running inference on a folder of images, an error occurred if not using --save-reconstructions since the batch size was inferred from reconstructions. Now batch size is inferred from the metadata, which will work for any combination of --save-reconstructions and --save-latents.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.51%. Comparing base (7cdb4fc) to head (ececb5a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #62   +/-   ##
=======================================
  Coverage   86.51%   86.51%           
=======================================
  Files          55       55           
  Lines        5022     5022           
=======================================
  Hits         4345     4345           
  Misses        677      677           

Files with missing lines	Coverage Δ
beast/inference.py	99.49% <100.00%> (ø)

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