[Diffusion] add GR00T-N1.7 pipeline with OpenPI serving (#3798)

Signed-off-by: Zhengyuan Su <su.zhengyuan@u.nus.edu>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

vllm_omni/diffusion/utils/flow_matching.py

@@ -0,0 +1,28 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import math
+
+import torch
+from torch import nn
+
+
+def swish(x: torch.Tensor) -> torch.Tensor:
+    return x * torch.sigmoid(x)
+
+
+class SinusoidalPositionalEncoding(nn.Module):
+    def __init__(self, embedding_dim: int) -> None:
+        super().__init__()
+        self.embedding_dim = embedding_dim
+
+    def forward(self, timesteps: torch.Tensor) -> torch.Tensor:
+        timesteps = timesteps.float()
+        B, T = timesteps.shape
+        device = timesteps.device
+
+        half_dim = self.embedding_dim // 2
+        exponent = -torch.arange(half_dim, dtype=torch.float, device=device) * (math.log(10000.0) / half_dim)
+        freqs = timesteps.unsqueeze(-1) * exponent.exp()  # (B, T, half_dim)
+        enc = torch.cat([torch.sin(freqs), torch.cos(freqs)], dim=-1)  # (B, T, embedding_dim)
+        return enc