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import torch
from typing import Optional, Tuple
def rotate_half(x):
x1, x2 = x.chunk(2, dim=-1)
return torch.cat((-x2, x1), dim=-1)
@torch.jit.script
def apply_rotary_pos_emb(x, cos, sin):
# Handle a possible sequence length mismatch in between q and k
cos = cos[:, :, : x.shape[-2], :]
sin = sin[:, :, : x.shape[-2], :]
return (x * cos) + (rotate_half(x) * sin)
class RotaryEmbedding(torch.nn.Module):
"""
Rotary position embeddings from RoFormer (Su et. al, 2021).
"""
def __init__(self, dim_model: int, *_, **__):
super().__init__()
# Generate and save the inverse frequency buffer (non trainable)
inv_freq = 1.0 / (10000 ** (torch.arange(0, dim_model, 2).float() / dim_model))
self.register_buffer("inv_freq", inv_freq)
self._seq_len_cached = None
self._cos_cached = None
self._sin_cached = None
def update_cos_sin_tables(self, x, seq_dimension=1):
seq_len = x.shape[seq_dimension]
# Reset the tables if the sequence length has changed,
# or if we're on a new device (possibly due to tracing for instance)
if (
seq_len != self._seq_len_cached
or self._cos_cached.device != x.device
or self._cos_cached.dtype != x.dtype
):
self._seq_len_cached = seq_len
t = torch.arange(
x.shape[seq_dimension], device=x.device, dtype=torch.float32
)
freqs = torch.einsum("i,j->ij", t, self.inv_freq.to(x.dtype))
emb = torch.cat((freqs, freqs), dim=-1).to(x.device)
self._cos_cached = emb.cos()[None, None, :, :].to(x.dtype)
self._sin_cached = emb.sin()[None, None, :, :].to(x.dtype)
return self._cos_cached, self._sin_cached
def forward(
self, q: torch.Tensor, k: torch.Tensor
) -> Tuple[torch.Tensor, torch.Tensor]:
self._cos_cached, self._sin_cached = self.update_cos_sin_tables(
k, seq_dimension=-2
)
return (
apply_rotary_pos_emb(q, self._cos_cached, self._sin_cached),
apply_rotary_pos_emb(k, self._cos_cached, self._sin_cached),
)
def __test_rope__():
dtype=torch.float16
batch=4
seqlen=2048
dim=4096
num_heads=32
dim_key_head=dim // num_heads
x=torch.randn(batch,seqlen,num_heads,dim_key_head).to(dtype=dtype).to('cuda')
rpe=RotaryEmbedding(dim_key_head).to(dtype=dtype).to('cuda')
q,k=rpe(q=x,k=x)
#__test_rope__()
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