Fix metadata dict support

[ethanwharris]

What does this PR do?

Fixes #384

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[codecov]

Codecov Report

Merging #393 (f0d5725) into master (c720807) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
+ Coverage   89.80%   89.82%   +0.02%     
==========================================
  Files          80       80              
  Lines        4207     4217      +10     
==========================================
+ Hits         3778     3788      +10     
  Misses        429      429              

Flag	Coverage Δ
gpu	?
pytest	?
unittests	89.82% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/core/data/batch.py	96.89% <100.00%> (+0.26%)	⬆️

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[tchaton]

LGTM !