boolean indexing of batched metatensor

[wyli]

Describe the bug

import torch
from monai.data import MetaTensor

data = MetaTensor(torch.zeros([2, 96, 96]))
data.is_batch = True

mask = torch.tensor([True, False])
print(data[mask].shape)

Traceback (most recent call last):
  File "/Users/Documents/MONAI/my_test_1.py", line 9, in <module>
    print(data[mask].shape)
  File "/Users/Documents/MONAI/monai/data/meta_tensor.py", line 297, in __torch_function__
    ret = MetaTensor.update_meta(ret, func, args, kwargs)
  File "/Users/Documents/MONAI/monai/data/meta_tensor.py", line 237, in update_meta
    ret_meta = decollate_batch(args[0], detach=False)[batch_idx]
TypeError: only integer tensors of a single element can be converted to an index