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| """ | |
| Linear Transformer proposed in "Transformers are RNNs: Fast Autoregressive Transformers with Linear Attention" | |
| Modified from: https://github.com/idiap/fast-transformers/blob/master/fast_transformers/attention/linear_attention.py | |
| """ | |
| import torch | |
| from torch.nn import Module, Dropout | |
| def elu_feature_map(x): | |
| return torch.nn.functional.elu(x) + 1 | |
| class LinearAttention(Module): | |
| def __init__(self, eps=1e-6): | |
| super().__init__() | |
| self.feature_map = elu_feature_map | |
| self.eps = eps | |
| def forward(self, queries, keys, values, q_mask=None, kv_mask=None): | |
| """Multi-Head linear attention proposed in "Transformers are RNNs" | |
| Args: | |
| queries: [N, L, H, D] | |
| keys: [N, S, H, D] | |
| values: [N, S, H, D] | |
| q_mask: [N, L] | |
| kv_mask: [N, S] | |
| Returns: | |
| queried_values: (N, L, H, D) | |
| """ | |
| Q = self.feature_map(queries) | |
| K = self.feature_map(keys) | |
| # set padded position to zero | |
| if q_mask is not None: | |
| Q = Q * q_mask[:, :, None, None] | |
| if kv_mask is not None: | |
| K = K * kv_mask[:, :, None, None] | |
| values = values * kv_mask[:, :, None, None] | |
| v_length = values.size(1) | |
| values = values / v_length # prevent fp16 overflow | |
| KV = torch.einsum("nshd,nshv->nhdv", K, values) # (S,D)' @ S,V | |
| Z = 1 / (torch.einsum("nlhd,nhd->nlh", Q, K.sum(dim=1)) + self.eps) | |
| queried_values = torch.einsum("nlhd,nhdv,nlh->nlhv", Q, KV, Z) * v_length | |
| return queried_values.contiguous() | |
| class FullAttention(Module): | |
| def __init__(self, use_dropout=False, attention_dropout=0.1): | |
| super().__init__() | |
| self.use_dropout = use_dropout | |
| self.dropout = Dropout(attention_dropout) | |
| def forward(self, queries, keys, values, q_mask=None, kv_mask=None): | |
| """Multi-head scaled dot-product attention, a.k.a full attention. | |
| Args: | |
| queries: [N, L, H, D] | |
| keys: [N, S, H, D] | |
| values: [N, S, H, D] | |
| q_mask: [N, L] | |
| kv_mask: [N, S] | |
| Returns: | |
| queried_values: (N, L, H, D) | |
| """ | |
| # Compute the unnormalized attention and apply the masks | |
| QK = torch.einsum("nlhd,nshd->nlsh", queries, keys) | |
| if kv_mask is not None: | |
| QK.masked_fill_( | |
| ~(q_mask[:, :, None, None] * kv_mask[:, None, :, None]).bool(), -1e9 | |
| ) | |
| # Compute the attention and the weighted average | |
| softmax_temp = 1.0 / queries.size(3) ** 0.5 # sqrt(D) | |
| A = torch.softmax(softmax_temp * QK, dim=2) | |
| if self.use_dropout: | |
| A = self.dropout(A) | |
| queried_values = torch.einsum("nlsh,nshd->nlhd", A, values) | |
| return queried_values.contiguous() | |