Dropout layer and register_forward_hook do not work together

fastai fastai dev
1

Does anyone know why the forward hooks do not register anything when you add a Dropout layer?

For instance in the AWD_LSTM module of fastai, there are the following layers:

class AWD_LSTM(Module):
    "AWD-LSTM inspired by https://arxiv.org/abs/1708.02182"
    initrange=0.1

    def __init__(self, vocab_sz, emb_sz, n_hid, n_layers, pad_token=1, hidden_p=0.2, input_p=0.6, embed_p=0.1,
                 weight_p=0.5, bidir=False):
        store_attr('emb_sz,n_hid,n_layers,pad_token')
        self.bs = 1
        self.n_dir = 2 if bidir else 1
        self.encoder = nn.Embedding(vocab_sz, emb_sz, padding_idx=pad_token)
        self.encoder_dp = EmbeddingDropout(self.encoder, embed_p)
        self.rnns = nn.ModuleList([self._one_rnn(emb_sz if l == 0 else n_hid, (n_hid if l != n_layers - 1 else emb_sz)//self.n_dir,
                                                 bidir, weight_p, l) for l in range(n_layers)])
        self.encoder.weight.data.uniform_(-self.initrange, self.initrange)
        self.input_dp = RNNDropout(input_p)
        self.hidden_dps = nn.ModuleList([RNNDropout(hidden_p) for l in range(n_layers)])
        self.reset()

I have changed the code to avoid using

EmbeddingDropout(self.encoder, embed_p)

and

RNNDropout(input_p)

and then the forward hooks work well. Why does this happen, anyone knows???

Is there any reason for the hooks not working with Dropout layers??