chore: vendor selva_core from jnwnlee/selva@d7d40a9

Pure PyTorch SelVA source for SelvaModelLoader/FeatureExtractor/Sampler nodes.
Imports rewritten from selva.* to selva_core.*. mel_converter.py: replaced
librosa.filters.mel with pure-numpy implementation to avoid librosa→numba→NumPy
version incompatibility in some ComfyUI environments.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

selva_core/model/embeddings.py

@@ -0,0 +1,49 @@
+import torch
+import torch.nn as nn
+
+# https://github.com/facebookresearch/DiT
+
+
+class TimestepEmbedder(nn.Module):
+    """
+    Embeds scalar timesteps into vector representations.
+    """
+
+    def __init__(self, dim, frequency_embedding_size, max_period):
+        super().__init__()
+        self.mlp = nn.Sequential(
+            nn.Linear(frequency_embedding_size, dim),
+            nn.SiLU(),
+            nn.Linear(dim, dim),
+        )
+        self.dim = dim
+        self.max_period = max_period
+        assert dim % 2 == 0, 'dim must be even.'
+
+        with torch.autocast('cuda', enabled=False):
+            self.freqs = nn.Buffer(
+                1.0 / (10000**(torch.arange(0, frequency_embedding_size, 2, dtype=torch.float32) /
+                               frequency_embedding_size)),
+                persistent=False)
+            freq_scale = 10000 / max_period
+            self.freqs = freq_scale * self.freqs
+
+    def timestep_embedding(self, t):
+        """
+        Create sinusoidal timestep embeddings.
+        :param t: a 1-D Tensor of N indices, one per batch element.
+                          These may be fractional.
+        :param dim: the dimension of the output.
+        :param max_period: controls the minimum frequency of the embeddings.
+        :return: an (N, D) Tensor of positional embeddings.
+        """
+        # https://github.com/openai/glide-text2im/blob/main/glide_text2im/nn.py
+
+        args = t[:, None].float() * self.freqs[None]
+        embedding = torch.cat([torch.cos(args), torch.sin(args)], dim=-1)
+        return embedding
+
+    def forward(self, t):
+        t_freq = self.timestep_embedding(t).to(t.dtype)
+        t_emb = self.mlp(t_freq)
+        return t_emb