fix: automatic round-down to quantum size

[kyujin-cho]

This PR resolves https://github.com/lablup/backend.ai-internal/issues/34.

[codecov]

Codecov Report

Merging #554 (7e980ae) into main (2fc081d) will increase coverage by 0.00%.
The diff coverage is 30.55%.

@@           Coverage Diff           @@
##             main     #554   +/-   ##
=======================================
  Coverage   49.33%   49.33%           
=======================================
  Files          55       55           
  Lines        8973     8991   +18     
=======================================
+ Hits         4427     4436    +9     
- Misses       4546     4555    +9     

Impacted Files	Coverage Δ
src/ai/backend/manager/models/image.py	49.13% <ø> (-0.26%)	⬇️
src/ai/backend/manager/scheduler/dispatcher.py	24.73% <0.00%> (-0.07%)	⬇️
src/ai/backend/manager/registry.py	18.29% <42.30%> (+0.72%)	⬆️

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