Fixed an issue when overwriting transform datasets

[AbhinavTuli]

The below code earlier gave an error "AttributeError: 'Dataset' object has no attribute '_chunk_commit_map'"

ds_transform = some_transform(iterable)
ds = ds_transform.store("abcd/ds_name")
ds = ds_transform.store("abcd/ds_name")

This has now been fixed.

[codecov]

Codecov Report

Merging #724 (fc864eb) into master (f688ec3) will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #724      +/-   ##
==========================================
+ Coverage   89.30%   89.54%   +0.23%     
==========================================
  Files          65       65              
  Lines        4423     4427       +4     
==========================================
+ Hits         3950     3964      +14     
+ Misses        473      463      -10     

Impacted Files	Coverage Δ
hub/api/dataset.py	91.70% <100.00%> (ø)
hub/store/metastore.py	91.89% <100.00%> (+6.84%)	⬆️
hub/store/s3_storage.py	88.52% <0.00%> (+4.91%)	⬆️

---

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 f688ec3...fc864eb. Read the comment docs.