Revert return type for Ops.alloc to Any (#785)

* Revert return type for Ops.alloc to Any

Because the stricter type led to mypy failures in spacy, revert the
changes for v8.1 and postpone to a later release.

* Fix import

thinc/backends/ops.py

@@ -1,7 +1,7 @@
 import math
 
 from typing import Optional, List, Tuple, Sequence, Type, Union, cast, TypeVar
-from typing import Iterator, overload
+from typing import Iterator, overload, Any
 import numpy
 import itertools
 
@@ -564,7 +564,7 @@ def alloc(
  *,
  dtype: Optional[DTypes] = "float32",
  zeros: bool = True,
- ) -> ArrayXd:
+ ) -> Any:
  """Allocate an array of a certain shape."""
  if isinstance(shape, int):
  shape = (shape,)