Adding yarn support. (#1099)

# What does this PR do?


Fixes #1017

Not sure if there's a mistake here but 

- NousResearch/Yarn-Llama-2-7b-128k seems to be working fine
- TheBloke/Yarn-Llama-2-13B-128K-GPTQ outputs garbage



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Fixes # (issue)


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server/text_generation_server/utils/layers.py

@@ -601,6 +601,19 @@ try:
                         device=inv_freq.device,
                         scaling_factor=scaling_factor,
                     )
+                elif rope_scaling["type"] == "yarn":
+                    return YarnPositionRotaryEmbedding(
+                        dim=2 * inv_freq.shape[0],
+                        max_position_embeddings=rope_scaling["original_max_position_embeddings"],
+                        base=10000.0,
+                        device=inv_freq.device,
+                        scaling_factor=scaling_factor,
+                        extrapolation_factor=1,
+                        attn_factor=1,
+                        beta_fast=32,
+                        beta_slow=1
+
+                    )
                 else:
                     raise NotImplementedError(
                         f"rope scaling type {rope_scaling['type']} is not implemented or invalid"
@@ -629,6 +642,19 @@ try:
                         device=inv_freq.device,
                         scaling_factor=scaling_factor,
                     )
+                elif rope_scaling["type"] == "yarn":
+                    return YarnPositionRotaryEmbedding(
+                        dim=2 * inv_freq.shape[0],
+                        max_position_embeddings=rope_scaling["original_max_position_embeddings"],
+                        base=10000.0,
+                        device=inv_freq.device,
+                        scaling_factor=scaling_factor,
+                        extrapolation_factor=1,
+                        attn_factor=1,
+                        beta_fast=32,
+                        beta_slow=1
+
+                    )
                 else:
                     raise NotImplementedError(
                         f"rope scaling type {rope_scaling['type']} is not implemented or invalid"
@@ -708,5 +734,76 @@ try:
                 self._cos_cached = torch.cos(freqs).to(dtype)
                 self._sin_cached = torch.sin(freqs).to(dtype)
 
+
+    # Inverse dim formula to find dim based on number of rotations
+    import math
+    def find_correction_dim(num_rotations, dim, base=10000, max_position_embeddings=2048):
+        return (dim * math.log(max_position_embeddings/(num_rotations * 2 * math.pi)))/(2 * math.log(base))
+
+    # Find dim range bounds based on rotations
+    def find_correction_range(low_rot, high_rot, dim, base=10000, max_position_embeddings=2048):
+        low = math.floor(find_correction_dim(
+            low_rot, dim, base, max_position_embeddings))
+        high = math.ceil(find_correction_dim(
+            high_rot, dim, base, max_position_embeddings))
+        return max(low, 0), min(high, dim-1)  # Clamp values just in case
+
+    def linear_ramp_mask(min, max, dim):
+        if min == max:
+            max += 0.001  # Prevent singularity
+
+        linear_func = (torch.arange(dim, dtype=torch.float32) - min) / (max - min)
+        ramp_func = torch.clamp(linear_func, 0, 1)
+        return ramp_func
+
+    def get_mscale(scale=1):
+        if scale <= 1:
+            return 1.0
+        return 0.1 * math.log(scale) + 1.0
+
+    class YarnPositionRotaryEmbedding(PositionRotaryEmbedding):
+        def __init__(self, dim, max_position_embeddings, base, device, scaling_factor,*, extrapolation_factor, attn_factor, beta_fast, beta_slow):
+            inv_freq = _create_inv_freq(dim, base, device)
+            super().__init__(inv_freq, scaling_factor)
+            self.dim = dim
+            self.max_position_embeddings = max_position_embeddings
+            self.base = base
+            self.extrapolation_factor = extrapolation_factor
+            self.attn_factor = attn_factor
+            self.beta_fast = beta_fast
+            self.beta_slow = beta_slow
+            self.mscale = float(get_mscale(self.scaling_factor) * self.attn_factor) # Get n-d magnitude scaling corrected for interpolation
+
+        def _update_cos_sin_cache(self, dtype, device, seqlen):
+            # Reset the tables if the sequence length has changed,
+            # or if we're on a new device (possibly due to tracing for instance)
+            if (
+                seqlen > self._seq_len_cached
+                or self._cos_cached.device != device
+                or self._cos_cached.dtype != dtype
+            ):
+                if seqlen > self.max_position_embeddings:
+                    inv_freq_extrapolation = _create_inv_freq(
+                        self.dim, self.base, self.inv_freq.device
+                    )
+                    freqs = 1.0 / inv_freq_extrapolation
+                    inv_freq_interpolation = 1.0 / (self.scaling_factor * freqs)
+                    low, high = find_correction_range(self.beta_fast, self.beta_slow, self.dim, self.base, self.max_position_embeddings)
+                    inv_freq_mask = (1 - linear_ramp_mask(low, high, self.dim // 2).float().to(device)) * self.extrapolation_factor # Get n-d rotational scaling corrected for extrapolation
+                    inv_freq = inv_freq_interpolation * (1 - inv_freq_mask) + inv_freq_extrapolation * inv_freq_mask
+
+                    self.inv_freq = inv_freq
+                    self.mscale = float(get_mscale(self.scaling_factor) * self.attn_factor) # Get n-d magnitude scaling corrected for interpolation
+
+
+                self._seq_len_cached = seqlen
+                t = torch.arange(seqlen, device=device, dtype=self.inv_freq.dtype)
+                # Don't do einsum, it converts fp32 to fp16
+                # freqs = torch.einsum("i,j->ij", t, self.inv_freq)
+
+                freqs = torch.outer(t, self.inv_freq.to(device=t.device))
+                self._cos_cached = (torch.cos(freqs) * self.mscale).to(dtype)
+                self._sin_cached = (torch.sin(freqs) * self.mscale).to(dtype)
+
 except ImportError:
     pass