code/model/modules/action_model/flow_matching_head/action_encoder.py

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# SPDX-License-Identifier: Apache-2.0
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
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#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
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import torch
import torch.nn as nn


def swish(x):
    return x * torch.sigmoid(x)


class SinusoidalPositionalEncoding(nn.Module):
    """
    Produces a sinusoidal encoding of shape (B, T, w)
    given timesteps of shape (B, T).
    """

    def __init__(self, embedding_dim):
        super().__init__()
        self.embedding_dim = embedding_dim

    def forward(self, timesteps):
        # timesteps: shape (B, T)
        # We'll compute sin/cos frequencies across dim T
        timesteps = timesteps.float()  # ensure float

        B, T = timesteps.shape
        device = timesteps.device

        half_dim = self.embedding_dim // 2
        # typical log space frequencies for sinusoidal encoding
        exponent = -torch.arange(half_dim, dtype=torch.float, device=device) * (
            torch.log(torch.tensor(10000.0)) / half_dim
        )
        # Expand timesteps to (B, T, 1) then multiply
        freqs = timesteps.unsqueeze(-1) * exponent.exp()  # (B, T, half_dim)

        sin = torch.sin(freqs)
        cos = torch.cos(freqs)
        enc = torch.cat([sin, cos], dim=-1)  # (B, T, w)

        return enc


class ActionEncoder(nn.Module):
    def __init__(self, action_dim, hidden_size):
        super().__init__()
        self.hidden_size = hidden_size

        # W1: R^{w x d}, W2: R^{w x 2w}, W3: R^{w x w}
        self.W1 = nn.Linear(action_dim, hidden_size)  # (d -> w)
        self.W2 = nn.Linear(2 * hidden_size, hidden_size)  # (2w -> w)
        self.W3 = nn.Linear(hidden_size, hidden_size)  # (w -> w)

        self.pos_encoding = SinusoidalPositionalEncoding(hidden_size)

    def forward(self, actions, timesteps):
        """
        actions:   shape (B, T, action_dim)
        timesteps: shape (B,)  -- a single scalar per batch item
        returns:   shape (B, T, hidden_size)
        """
        B, T, _ = actions.shape

        # 1) Expand each batch's single scalar time 'tau' across all T steps
        #    so that shape => (B, T)
        #    e.g. if timesteps is (B,), replicate across T
        if timesteps.dim() == 1 and timesteps.shape[0] == B:
            # shape (B,) => (B,T)
            timesteps = timesteps.unsqueeze(1).expand(-1, T)
        else:
            raise ValueError(
                "Expected `timesteps` to have shape (B,) so we can replicate across T."
            )

        # 2) Standard action MLP step for shape => (B, T, w)
        a_emb = self.W1(actions)

        # 3) Get the sinusoidal encoding (B, T, w)
        tau_emb = self.pos_encoding(timesteps).to(dtype=a_emb.dtype)

        # 4) Concat along last dim => (B, T, 2w), then W2 => (B, T, w), swish
        x = torch.cat([a_emb, tau_emb], dim=-1)
        x = swish(self.W2(x))

        # 5) Finally W3 => (B, T, w)
        x = self.W3(x)

        return x