Cannot create compose on SqueezeTransform, UnsqueezeTransform and FlattenTransform

[altre]

Some transforms assume to already have a parent when calling set_parent(), in which case creating a new Compose with them fails:

>>> Compose(t)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/alanschelten/rl/torchrl/envs/transforms/transforms.py", line 630, in __init__
    t.set_parent(self)
  File "/Users/alanschelten/rl/torchrl/envs/transforms/transforms.py", line 1115, in set_parent
    batch_size = parent.batch_size
AttributeError: 'NoneType' object has no attribute 'batch_size'
>>> t = UnsqueezeTransform(0)
>>> Compose(t)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/alanschelten/rl/torchrl/envs/transforms/transforms.py", line 630, in __init__
    t.set_parent(self)
  File "/Users/alanschelten/rl/torchrl/envs/transforms/transforms.py", line 1115, in set_parent
    batch_size = parent.batch_size
AttributeError: 'NoneType' object has no attribute 'batch_size'
>>> t = FlattenObservation()
>>> Compose(t)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/alanschelten/rl/torchrl/envs/transforms/transforms.py", line 630, in __init__
    t.set_parent(self)
  File "/Users/alanschelten/rl/torchrl/envs/transforms/transforms.py", line 1045, in set_parent
    observation_spec = self.parent.observation_spec
  File "/Users/alanschelten/rl/torchrl/envs/transforms/transforms.py", line 241, in parent
    raise ValueError(
ValueError: Compose parent was of type <class 'NoneType'> but expected TransformedEnv.