Make CachedSimulatedDataset mappable Dataset

[zhixiangteoh]

Partially fix bug where each epoch runs num_workers * n_batches + valid_n_batches iterations when it should be n_batches + valid_n_batches iterations. Partial because bug would remain for SimulatedDataset.

Associated sanity-check test results ensuring similar validation losses / speed of the two approaches:

dataset type	mappable	mappable	iterable	iterable
num workers	0	32	0	32
training loss (after 250 iterations)	1.82	1.82	1.92	1.9
val loss	1.8513	1.8513	1.90651	-
speed (median it/s)	4.98	1.4	4.92	3.98

[codecov]

Codecov Report

Merging #760 (ff2f6f0) into master (3cd181c) will increase coverage by 0.21%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #760      +/-   ##
==========================================
+ Coverage   88.53%   88.75%   +0.21%     
==========================================
  Files          14       14              
  Lines        1265     1254      -11     
==========================================
- Hits         1120     1113       -7     
+ Misses        145      141       -4     

Flag	Coverage Δ
unittests	88.75% <94.44%> (+0.21%)	⬆️

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

Impacted Files	Coverage Δ
bliss/simulator/simulated_dataset.py	89.47% <92.85%> (-0.82%)	⬇️
bliss/generate.py	98.52% <100.00%> (+5.57%)	⬆️

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[jeff-regier]

Looks good!