Add iterable behavior for PointerTensor

[gmuraru]

Pull Request

Fixes #3543

Description

Add the possibility to do:

for tensor in ptr:
     print(tensor) # should return a new pointer tensor

Affected Dependencies

List any dependencies that are required for this change.

Type of Change

Please mark options that are relevant.

• Bug fix (non-breaking change which fixes an issue)
• Documentation (non-breaking change which adds documentation)
• Improvement (non-breaking change that improves the performance or reliability of existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How has this been tested?

• Test if the number of tensors alice has
• Provide instructions so we can reproduce.
• List any relevant details for your test configuration.

Checklist

• I did follow the Contribution Guidelines
• I did follow the Code of Conduct
• I have commented my code following the OpenMined Styleguide
• I have added tests for my changes

Additional Context

Add a new method to the storage object such that we can call len(alice.object_store and will return the number of objects from the worker.

[karlhigley]

🚀

[codecov]

Codecov Report

Merging #3659 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3659   +/-   ##
=======================================
  Coverage   94.74%   94.75%           
=======================================
  Files         182      182           
  Lines       18036    18061   +25     
=======================================
+ Hits        17089    17114   +25     
  Misses        947      947           

Impacted Files	Coverage Δ
test/generic/pointers/test_callable_pointer.py	100.00% <ø> (ø)
test/generic/pointers/test_dataset_pointer.py	100.00% <ø> (ø)
test/generic/pointers/test_multi_pointer.py	100.00% <ø> (ø)
test/generic/pointers/test_pointer_plan.py	100.00% <ø> (ø)
test/generic/test_autograd.py	100.00% <ø> (ø)
test/generic/test_functions.py	100.00% <ø> (ø)
test/generic/test_gc.py	100.00% <ø> (ø)
test/generic/test_logging.py	100.00% <ø> (ø)
test/generic/test_private.py	100.00% <ø> (ø)
syft/generic/frameworks/hook/hook_args.py	95.49% <100.00%> (ø)
... and 13 more

[gmuraru]

@karlhigley are you also okay with this change? to be able to call len on the object store

[karlhigley]

Yeah, took me a minute to see what it was for, but seems pretty convenient. I think there's a count_objects() method on BaseWorker that could use this, but that's a "nice to have tweak" that shouldn't block this PR.