Don't implicitly chain exceptions when accessing non-existent storage attributes

[akihironitta]

This PR changes the traceback when a user tries to access attributes on Data that doesn't exist:

import torch_geometric

data = torch_geometric.data.Data()
data.asdf

Before

Traceback (most recent call last):
  File "/workspaces/pytorch_geometric/torch_geometric/data/storage.py", line 80, in __getattr__
    return self[key]
  File "/workspaces/pytorch_geometric/torch_geometric/data/storage.py", line 105, in __getitem__
    return self._mapping[key]
KeyError: 'asdf'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/workspaces/pytorch_geometric/test.py", line 4, in <module>
    data.asdf
  File "/workspaces/pytorch_geometric/torch_geometric/data/data.py", line 475, in __getattr__
    return getattr(self._store, key)
  File "/workspaces/pytorch_geometric/torch_geometric/data/storage.py", line 82, in __getattr__
    raise AttributeError(
AttributeError: 'GlobalStorage' object has no attribute 'asdf'

After

Traceback (most recent call last):
  File "/workspaces/pytorch_geometric/test.py", line 4, in <module>
    data.asdf
  File "/workspaces/pytorch_geometric/torch_geometric/data/data.py", line 475, in __getattr__
    return getattr(self._store, key)
  File "/workspaces/pytorch_geometric/torch_geometric/data/storage.py", line 82, in __getattr__
    raise AttributeError(
AttributeError: 'GlobalStorage' object has no attribute 'asdf'

Alternative

By explicitly chaining exceptions, one of the lines in the error changes:

-       except KeyError:
+       except KeyError as e:
            raise AttributeError(
-               f"'{self.__class__.__name__}' object has no attribute '{key}'")
+               f"'{self.__class__.__name__}' object has no attribute '{key}'") from e

- During handling of the above exception, another exception occurred:
+ The above exception was the direct cause of the following exception:

[codecov]

Codecov Report

Merging #7734 (e0a6800) into master (9bc7017) will decrease coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7734      +/-   ##
==========================================
- Coverage   91.93%   91.58%   -0.36%     
==========================================
  Files         452      452              
  Lines       25537    25534       -3     
==========================================
- Hits        23478    23385      -93     
- Misses       2059     2149      +90     

Impacted Files	Coverage Δ
torch_geometric/data/storage.py	83.11% <ø> (ø)

... and 20 files with indirect coverage changes

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[mananshah99]

Generally prefer chaining exceptions, but since this is a very user-facing attribute, the solution here seems very reasonable. Thanks!