How do I tie embeddings between a `torch.nn.Embedding` and `torch.nn.EmbeddingBag`?
I'd like to tie the embedding layers between two parts of my neural network: one which embeds tokens where order matters (i.e., nn.Embedding) and one which embeds tokens where order doesn't matter (i.e., nn.EmbeddingBag). I ran into numerical stability issues when creating my own EmbeddingBag-like object and doing the reduction myself so I'd like to use the officially support nn.EmbeddingBag; however, it seems like my attempt to tie weights (below) doesn't work
#!/usr/bin/env python3

import torch
import torch.nn as nn

if __name__ == &quot;__main__&quot;:
    V, max_seq, padding_idx, emb_dim, B = 10, 100, 1, 512, 32

    # Create an embedding layer and initialize an embeddingbag with those weights
    emb_layer = nn.Embedding(V, emb_dim, padding_idx=padding_idx)
    emb_bag = nn.EmbeddingBag.from_pretrained(emb_layer.weight, freeze=False, padding_idx=padding_idx)

    tokens = torch.randint(0, V, (B, max_seq))
    y = torch.randn((B, emb_dim))
    loss = nn.MSELoss()

    # backprop through the embedding bag
    y_ = emb_bag(tokens)
    l = loss(y_, y)
    assert emb_bag.weight.grad is None
    l.backward()
    assert emb_bag.weight.grad is not None

    # if we're tying weights, backpropping through the emb bag should
    # yield the same gradients in the embedding layer, but... the following assertion fails
    assert emb_layer.weight.grad is not None and \
        torch.allclose(emb_bag.weight.grad, emb_layer.weight.grad)

Is there some way to tie the weights in both or do I need to be creative with how I emulate the embeddingbag behavior?

from_pretrained() will copy the weights: emb_layer.weight is emb_bag.weight will be False.
You can just set the weight attribute directly:
emb_bag.weight = emb_layer.weight
