Fix/cloud dataset bugs

[verbose-void]

🚀 🚀 Pull Request

Checklist:

• My code follows the style guidelines of this project and the Contributing document
• I have commented my code, particularly in hard-to-understand areas
• I have kept the coverage-rate up
• I have performed a self-review of my own code and resolved any problems
• I have checked to ensure there aren't any other open Pull Requests for the same change
• I have described and made corresponding changes to the relevant documentation
• New and existing unit tests pass locally with my changes

Changes

fixes the

sample = ds[0]

bug when ds is a hub cloud dataset

also fixes the "cannot overwrite dataset" bug, again for hub cloud datasets.

added mnist test

[codecov]

Codecov Report

Merging #1294 (1e4a604) into main (c1f908e) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1294      +/-   ##
==========================================
+ Coverage   91.27%   91.32%   +0.04%     
==========================================
  Files         141      142       +1     
  Lines        9862     9871       +9     
==========================================
+ Hits         9002     9015      +13     
+ Misses        860      856       -4     

Impacted Files	Coverage Δ
hub/api/dataset.py	95.23% <100.00%> (ø)
hub/api/tests/test_hosted_datasets.py	100.00% <100.00%> (ø)
hub/core/dataset/__init__.py	86.66% <100.00%> (ø)
hub/core/dataset/dataset.py	92.97% <100.00%> (+0.05%)	⬆️
hub/core/dataset/hub_cloud_dataset.py	81.63% <100.00%> (ø)
hub/cli/list_datasets.py	81.81% <0.00%> (-18.19%)	⬇️
hub/client/utils.py	67.21% <0.00%> (-14.76%)	⬇️
hub/util/exceptions.py	80.57% <0.00%> (-0.36%)	⬇️
hub/core/meta/tensor_meta.py	88.96% <0.00%> (+0.68%)	⬆️
hub/util/storage.py	96.49% <0.00%> (+3.50%)	⬆️
... and 2 more

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