[dataloader] type annotation is broken

[ssnl]

In #19228 I did not update the type annotations. We should fix this.

[utikeev]

I've also found that bunch of optional parameters are not labeled with Optional:
dataloader.pyi:

def __init__(self, dataset: Dataset[T_co], batch_size: int=..., shuffle: bool=..., sampler: Sampler[int]=...,
                 num_workers: int=..., collate_fn: _collate_fn_t=..., pin_memory: bool=...,
                 drop_last: bool=..., timeout: float=..., worker_init_fn: _worker_init_fn_t=...) -> None: ...

def __init__(self, dataset: Dataset[T_co], batch_sampler: Sampler[Sequence[int]]=..., num_workers: int=...,
                 collate_fn: _collate_fn_t=..., pin_memory: bool=..., timeout: float=...,
                 worker_init_fn: _worker_init_fn_t=...) -> None: ...

dataloader.py:

def __init__(self, dataset, batch_size=1, shuffle=False, sampler=None,
                 batch_sampler=None, num_workers=0, collate_fn=default_collate,
                 pin_memory=False, drop_last=False, timeout=0,
                 worker_init_fn=None):

So these are sampler, batch_sampler and worker_init_fn.

[ssnl]

I think this is fixed by a couple recent PRs