[model/predict] Add support for different types of action outputs

[ANarayan]

This PR adds support processing action outputs of different types. The current implementation expects that all action outputs are of type tensor. Here, we expand support for outputs of type dict. This PR also adds a new utils function (merge_objects) which merges different types of objects.

[codecov[bot]]

Codecov Report

Merging #113 (7f76dbf) into master (03c80bc) will increase coverage by 0.10%. The diff coverage is 96.66%.

:exclamation: Current head 7f76dbf differs from pull request most recent head 044ed8c. Consider uploading reports for the commit 044ed8c to get more accurate results

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   91.21%   91.31%   +0.10%     
==========================================
  Files          40       40              
  Lines        1991     2039      +48     
  Branches      425      446      +21     
==========================================
+ Hits         1816     1862      +46     
  Misses        101      101              
- Partials       74       76       +2     

Flag	Coverage Δ
unittests	91.31% <96.66%> (+0.10%)	:arrow_up:

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Impacted Files	Coverage Δ
src/emmental/meta.py	84.46% <ø> (ø)
src/emmental/utils/parse_args.py	99.31% <ø> (ø)
src/emmental/utils/utils.py	96.70% <95.91%> (-0.38%)	:arrow_down:
src/emmental/logging/checkpointer.py	93.33% <100.00%> (ø)
src/emmental/model.py	88.46% <100.00%> (+0.11%)	:arrow_up: