[Feature] Support saving and loading npz file in offline evaluation mode.

[LeoXing1996]

Time cost comparison

Model	Offline	Offline w save_npz	online
SAGAN-128	14:56.74	6:57.43	5:19.90

This feature can greatly speed up sampling result saving and metric evaluation.

[codecov]

Codecov Report

Merging #201 (7ca9b49) into master (55c9a42) will decrease coverage by 0.33%.
The diff coverage is 49.19%.

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
- Coverage   75.60%   75.27%   -0.34%     
==========================================
  Files         128      129       +1     
  Lines        8933     9051     +118     
  Branches     1783     1807      +24     
==========================================
+ Hits         6754     6813      +59     
- Misses       1729     1786      +57     
- Partials      450      452       +2     

Flag	Coverage Δ
unittests	75.27% <49.19%> (-0.34%)	⬇️

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

Impacted Files	Coverage Δ
mmgen/core/evaluation/evaluation.py	7.78% <6.25%> (-0.34%)	⬇️
mmgen/datasets/file_dataset.py	94.64% <94.64%> (ø)
mmgen/datasets/__init__.py	100.00% <100.00%> (ø)
mmgen/datasets/pipelines/formatting.py	98.24% <100.00%> (+0.03%)	⬆️
...chitectures/stylegan/generator_discriminator_v1.py	88.70% <0.00%> (-1.13%)	⬇️
...chitectures/stylegan/generator_discriminator_v2.py	87.77% <0.00%> (+1.11%)	⬆️

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[LeoXing1996]

Defaults to False

[plyfager]

A description to clarify why you pull this request is supposed to be offered. Also, our documentation should be updated accordingly.