Proposed changes

Modified the error message logic of einsum() mentioned on #2677 to clearly identify which operand is problematic, show its shape and the specific subscript pattern that's causing the issue.

import mlx.core as mx
y = mx.ones((6, 10))
x = mx.ones((6,))
mx.einsum("...i,ij->...j", x[0], y)

New Error Message

Traceback (most recent call last):
  File "/Users/mv/repos/mlx/einsum_test.py", line 4, in <module>
    mx.einsum("...i,ij->...j", x[0], y)
ValueError: [einsum] Operand 0 with shape {} has insufficient dimensions for subscript '...i'.
The ellipsis requires at least 1 dimensions but the operand has 0 dimensions.

Example on the same error in PyTorch:
einsum(): the number of subscripts in the equation (1) is more than the number of dimensions (0) for operand 0

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works -> Not necessary
• I have updated the necessary documentation (if needed) -> Not necessary