Misc update to models

[merrymercy]

1. Fix minor bugs
2. Print batch number during training
3. Add 'SelectFixed' and 'SequenceFixed' to random and dfs strategy
4. Add more interfaces to IndependentOperatorModel to support continuous training from saved models.

[codecov-io]

Codecov Report

Merging #6 into master will increase coverage by 0.57%. The diff coverage is 62.02%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   80.64%   81.21%   +0.57%     
==========================================
  Files          49       49              
  Lines        6183     6224      +41     
==========================================
+ Hits         4986     5055      +69     
+ Misses       1197     1169      -28

Impacted Files	Coverage Δ
atlas/models/__init__.py	100% <100%> (ø)	:arrow_up:
atlas/models/tensorflow/graphs/gnn.py	71.92% <100%> (+1.02%)	:arrow_up:
atlas/models/tensorflow/graphs/earlystoppers.py	84.84% <100%> (+0.47%)	:arrow_up:
atlas/__init__.py	100% <100%> (ø)	:arrow_up:
atlas/utils/astutils.py	64.58% <100%> (+0.75%)	:arrow_up:
atlas/generators.py	89.5% <100%> (ø)	:arrow_up:
atlas/models/tensorflow/graphs/operators.py	97.91% <100%> (ø)	:arrow_up:
atlas/models/imitation.py	28.18% <21.87%> (+28.18%)	:arrow_up:
atlas/strategies/randomized.py	86.48% <66.66%> (-3.84%)	:arrow_down:
atlas/strategies/dfs.py	83.05% <75%> (-0.88%)	:arrow_down:
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 3f07a8f...2d8247b. Read the comment docs.

[rbavishi]

I added some comments. The refactorings and minor bug-fixes look good and can be ported to the latest version.

[merrymercy]

Close this for now. I will send a new PR after rebasing to current master