allow batch_list to batch tensors with different children

[jsalotti]

Add an argument to aloscene.batch_list, allowing to batch tensors with different children or properties with different values.
Closes #272

• New feature 1 : Merge with different properties

f1 = Frame(torch.ones(3,10,10))
f2 = Frame(torch.ones(3,10,10))
f1.baseline = 0.5
f2.baseline = 1
f = aloscene.batch_list([f1, f2], intersection=True)
print(f.baseline)

Here, the baseline property has different values for different tensors. Therefore, the property baseline of the batched tensors has baseline set to None.

• New feature 2 : Merge with different mergeable children

f1 = Frame(torch.ones(3,10,10))
f2 = Frame(torch.ones(3,10,10))
f1.append_disparity(Disparity(torch.ones(1,10,10)))
f = aloscene.batch_list([f1, f2], intersection=True)
print(f.disparity)

Only one of the two frames has a disparity child, therefore the batched tensor will have disparity set to None

Note: this will not drop unmergeable children, because by definition they allow that some tensors have None values for a child before merging

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This pull request includes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

[thibo73800]

Can you provide this PR with a new set of unit test to check compatibility with previous behavior + exposing the different new possible scenarios ?

[Data-Iab]

Tested.
Will be merged once @thibo73800 approves

[jsalotti]

@Data-Iab Unittests have been added ; and an error fixed. The branch is now ready to be merged.