add embedding bag skeleton take 2 (pytorch#61126)

Summary:
Pull Request resolved: pytorch#61126

adding skeleton implementations of quantized embedding tables with zeroes

Test Plan:
compilation, farm test, and ran test_find_dangling_impls and passed

did a manual negative test and verified the message is printed properly
```
======================================================================
FAIL: test_find_dangling_impls (test_dispatch.TestPythonDispatcher)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/data/users/hyz/fbsource/fbcode/buck-out/opt/gen/caffe2/test/others#binary,link-tree/test_dispatch.py", line 892, in test_find_dangling_impls
    self.assertEqual(
  File "/data/users/hyz/fbsource/fbcode/buck-out/opt/gen/caffe2/test/others#binary,link-tree/torch/testing/_internal/common_utils.py", line 1498, in assertEqual
    super().assertTrue(result, msg=self._get_assert_msg(msg, debug_msg=debug_msg))
AssertionError: False is not true : Scalars failed to compare as equal! 0 != 1
Expect zero dangling impls, but found: ['name: quantized::qembedding_bag_4bit_unpack\nschema: (none)\nCUDA: registered at caffe2/aten/src/ATen/native/quantized/cuda/embedding_bag.cu:394 :: (Tensor _0) -> (Tensor _0) [ boxed unboxed ]\n']

Reviewed By: walterddr

Differential Revision: D29518274

fbshipit-source-id: d0cb81c8bf51cdc4b83038758131ccf61e4360f5

Author: Hector Yuen