sensor_size default to None on some transforms

[HussainMSajwani]

ToFrame and DropEvent and their functional counterparts handle sensor_size being None internally. I have changed those two to accept sensor_size being None. This is useful for datasets where sensor_size is not given such as NCaltech101.

Some other transforms technically do not need sensor_size but we can handle those in a future pull request.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (debdd2e) 76.98% compared to head (67bf7cd) 77.01%.

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@@             Coverage Diff             @@
##           develop     #277      +/-   ##
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+ Coverage    76.98%   77.01%   +0.03%     
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  Files           54       54              
  Lines         3059     3059              
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+ Hits          2355     2356       +1     
+ Misses         704      703       -1     

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[biphasic]

Here we need to explain what setting it to None means, that it'll automatically calculate the sensor size from the maximum event locations in the data and that that is not a guarantee that it is the true sensor size

[biphasic]

The problem with calculating the sensor size automatically every time is that users will omit the parameter because it works most of the times. But then sometimes, not all the samples in a batch will have the same calculated sensor size. During batching, this will throw an error saying that it can only stack tensors of the same size. I think I prefer that the user has to be explicit about this

[HussainMSajwani]

The reason I created this pull request is that I was trying to use some of the transforms on N-Caltech10, which has variable size, and most expected a sensor_size argument. This makes the transforms not usable on N-Caltech101.

Some transforms already handle missing sensor_size internally. See for example ToFrame. But still an error is thrown when sensor_size is not passed. This pull request only allows sensor_size not to be passed.

Perhaps a warning should be displayed to the user when the sensor_size is not declared explicitly. This allows the transforms to work on N-Caltech101 and still alerts the user of potential errors downstream coming from non fixed sensor_size. I can help making this a thing if you agree.