model/rotary.py

import torch
from torch import nn


class Rotary(torch.nn.Module):
    def __init__(self, dim, base=10_000):
        super().__init__()
        inv_freq = 1.0 / (base ** (torch.arange(0, dim, 2).float() / dim))
        self.register_buffer("inv_freq", inv_freq)
        self.seq_len_cached = None
        self.cos_cached = None
        self.sin_cached = None

    def forward(self, x, seq_dim=1):
        seq_len = x.shape[seq_dim]
        if seq_len != self.seq_len_cached:
            self.seq_len_cached = seq_len
            t = torch.arange(x.shape[seq_dim], device=x.device).type_as(self.inv_freq)
            freqs = torch.einsum("i,j->ij", t, self.inv_freq.clone())
            emb = torch.cat((freqs, freqs), dim=-1).to(x.device)
            # dims are: batch, seq_len, qkv, head, dim
            self.cos_cached = emb.cos()[None, :, None, None, :].repeat(1,1,3,1,1)
            self.sin_cached = emb.sin()[None, :, None, None, :].repeat(1,1,3,1,1)
            # This makes the transformation on v an identity.
            self.cos_cached[:,:,2,:,:].fill_(1.)
            self.sin_cached[:,:,2,:,:].fill_(0.)

        return self.cos_cached, self.sin_cached


def rotate_half(x):
    half = x.size(-1) // 2
    x1, x2 = x[..., :half], x[..., half:]
    return torch.cat(
        (-x2, x1), dim=-1
    )


def _apply_rotary_pos_emb_torch(qkv, cos, sin):
    return (qkv * cos) + (rotate_half(qkv) * sin)


def apply_rotary_pos_emb(qkv, cos, sin):
    return _apply_rotary_pos_emb_torch(qkv, cos, sin)