Fix a bug when re-assigning the same sample decoder to a task encoder through legacy args

[philipp-fischer]

Fixes a bug that can occur when using a task encoder as shown below.
The error message would be AssertionError: Task encoder already has a decoder, and setting using deprecated kwargs is not allowed.

Example:

te = MyTaskEncoder()

dataset = get_train_dataset(
    path="./metadat2.yaml",
    ...
    task_encoder=te,
    image_decode="pil",
)

val_dataset = get_val_dataset(
    path="./metadat3.yaml",
    ...
    task_encoder=te,
    image_decode="pil",
)