Summary:
Currently, it is not possible to access a sub-transform using an indexer for all 3d transforms inheriting the `Transforms3d` class.
For instance:
```python
from pytorch3d import transforms
N = 10
r = transforms.random_rotations(N)
T = transforms.Transform3d().rotate(R=r)
R = transforms.Rotate(r)
x = T[0] # ok
x = R[0] # TypeError: __init__() got an unexpected keyword argument 'matrix'
```
This is because all these classes (namely `Rotate`, `Translate`, `Scale`, `RotateAxisAngle`) inherit the `__getitem__()` method from `Transform3d` which has the [following code on line 201](https://github.com/facebookresearch/pytorch3d/blob/main/pytorch3d/transforms/transform3d.py#L201):
```python
return self.__class__(matrix=self.get_matrix()[index])
```
The four classes inheriting `Transform3d` are not initialized through a matrix argument, hence they error.
I propose to modify the `__getitem__()` method of the `Transform3d` class to fix this behavior. The least invasive way to do it I can think of consists of creating an empty instance of the current class, then setting the `_matrix` attribute manually. Thus, instead of
```python
return self.__class__(matrix=self.get_matrix()[index])
```
I propose to do:
```python
instance = self.__class__.__new__(self.__class__)
instance._matrix = self.get_matrix()[index]
return instance
```
As far as I can tell, this modification occurs no modification whatsoever for the user, except for the ability to index all 3d transforms.
Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/1801
Reviewed By: MichaelRamamonjisoa
Differential Revision: D58410389
Pulled By: bottler
fbshipit-source-id: f371e4c63d2ae4c927a7ad48c2de8862761078de
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