fix: match torch frontend unflatten signature with native torch

as seen at https://pytorch.org/docs/stable/generated/torch.unflatten.html

ivy/functional/frontends/torch/miscellaneous_ops.py

@@ -555,7 +555,7 @@ def triu_indices(row, col, offset=0, dtype="int64", device="cpu", layout=None):
 
 
 @to_ivy_arrays_and_back
-def unflatten(input, /, *, dim, sizes):
+def unflatten(input, dim, sizes):
     return ivy.unflatten(input, dim=dim, shape=sizes, out=None)