"""Deserialize a base64 encoded string or bytes-like object into bytes.

This function handles both bytes-like objects and base64 encoded strings. If the input is already a bytes-like object, it is returned as is. For base64 encoded strings, the function ensures proper padding and converts URL-safe characters to standard base64 characters before decoding.

Args:
    attr (Union[bytes, bytearray, str]): The input to deserialize. Can be a bytes-like object or a base64 encoded string.

Returns:
    bytes: The deserialized bytes object.
"""
You should write code starting with:
import base64
from typing import Union

def _deserialize_bytes_base64(attr: Union[bytes, bytearray, str]) -> bytes:
    # TODO: Implement this function
    pass

