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# Copyright (c) 2025, NVIDIA CORPORATION. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""
GPT components for the NeMo Models tutorial.
This module contains the neural network components used in the tutorial 01_NeMo_Models.ipynb
"""
import math
from typing import Optional
import torch
import torch.nn as nn
from torch.nn import functional as F
from nemo.core import NeuralModule, typecheck
from nemo.core.neural_types import EmbeddedTextType, EncodedRepresentation, Index, LogitsType, NeuralType
from nemo.core.neural_types.elements import *
# Custom Element Types
class AttentionType(EncodedRepresentation):
"""Basic Attention Element Type"""
class SelfAttentionType(AttentionType):
"""Self Attention Element Type"""
class CausalSelfAttentionType(SelfAttentionType):
"""Causal Self Attention Element Type"""
# Neural Network Modules (not NeMo neural modules)
class CausalSelfAttention(nn.Module):
"""
A vanilla multi-head masked self-attention layer with a projection at the end.
It is possible to use torch.nn.MultiheadAttention here but I am including an
explicit implementation here to show that there is nothing too scary here.
"""
def __init__(self, n_embd, block_size, n_head, attn_pdrop, resid_pdrop):
super().__init__()
assert n_embd % n_head == 0
self.n_head = n_head
# key, query, value projections for all heads
self.key = nn.Linear(n_embd, n_embd)
self.query = nn.Linear(n_embd, n_embd)
self.value = nn.Linear(n_embd, n_embd)
# regularization
self.attn_drop = nn.Dropout(attn_pdrop)
self.resid_drop = nn.Dropout(resid_pdrop)
# output projection
self.proj = nn.Linear(n_embd, n_embd)
# causal mask to ensure that attention is only applied to the left in the input sequence
self.register_buffer("mask", torch.tril(torch.ones(block_size, block_size)).view(1, 1, block_size, block_size))
def forward(self, x, layer_past=None):
B, T, C = x.size()
# calculate query, key, values for all heads in batch and move head forward to be the batch dim
k = self.key(x).view(B, T, self.n_head, C // self.n_head).transpose(1, 2) # (B, nh, T, hs)
q = self.query(x).view(B, T, self.n_head, C // self.n_head).transpose(1, 2) # (B, nh, T, hs)
v = self.value(x).view(B, T, self.n_head, C // self.n_head).transpose(1, 2) # (B, nh, T, hs)
# causal self-attention; Self-attend: (B, nh, T, hs) x (B, nh, hs, T) -> (B, nh, T, T)
att = (q @ k.transpose(-2, -1)) * (1.0 / math.sqrt(k.size(-1)))
att = att.masked_fill(self.mask[:, :, :T, :T] == 0, float('-inf'))
att = F.softmax(att, dim=-1)
att = self.attn_drop(att)
y = att @ v # (B, nh, T, T) x (B, nh, T, hs) -> (B, nh, T, hs)
y = y.transpose(1, 2).contiguous().view(B, T, C) # re-assemble all head outputs side by side
# output projection
y = self.resid_drop(self.proj(y))
return y
class Block(nn.Module):
"""an unassuming Transformer block"""
def __init__(self, n_embd, block_size, n_head, attn_pdrop, resid_pdrop):
super().__init__()
self.ln1 = nn.LayerNorm(n_embd)
self.ln2 = nn.LayerNorm(n_embd)
self.attn = CausalSelfAttention(n_embd, block_size, n_head, attn_pdrop, resid_pdrop)
self.mlp = nn.Sequential(
nn.Linear(n_embd, 4 * n_embd),
nn.GELU(),
nn.Linear(4 * n_embd, n_embd),
nn.Dropout(resid_pdrop),
)
def forward(self, x):
x = x + self.attn(self.ln1(x))
x = x + self.mlp(self.ln2(x))
return x
# NeMo Neural Modules
class GPTEmbedding(NeuralModule):
def __init__(self, vocab_size: int, n_embd: int, block_size: int, embd_pdrop: float = 0.0):
super().__init__()
# input embedding stem: drop(content + position)
self.tok_emb = nn.Embedding(vocab_size, n_embd)
self.pos_emb = nn.Parameter(torch.zeros(1, block_size, n_embd))
self.drop = nn.Dropout(embd_pdrop)
@typecheck()
def forward(self, idx):
b, t = idx.size()
# forward the GPT model
token_embeddings = self.tok_emb(idx) # each index maps to a (learnable) vector
position_embeddings = self.pos_emb[:, :t, :] # each position maps to a (learnable) vector
x = self.drop(token_embeddings + position_embeddings)
return x
@property
def input_types(self):
return {'idx': NeuralType(('B', 'T'), Index())}
@property
def output_types(self):
return {'embeddings': NeuralType(('B', 'T', 'C'), EmbeddedTextType())}
class GPTTransformerEncoder(NeuralModule):
def __init__(
self,
n_embd: int,
block_size: int,
n_head: int,
n_layer: int,
attn_pdrop: float = 0.0,
resid_pdrop: float = 0.0,
):
super().__init__()
self.blocks = nn.Sequential(
*[Block(n_embd, block_size, n_head, attn_pdrop, resid_pdrop) for _ in range(n_layer)]
)
@typecheck()
def forward(self, embed):
return self.blocks(embed)
@property
def input_types(self):
return {'embed': NeuralType(('B', 'T', 'C'), EmbeddedTextType())}
@property
def output_types(self):
return {'encoding': NeuralType(('B', 'T', 'C'), CausalSelfAttentionType())}
class GPTDecoder(NeuralModule):
def __init__(self, n_embd: int, vocab_size: int):
super().__init__()
self.ln_f = nn.LayerNorm(n_embd)
self.head = nn.Linear(n_embd, vocab_size, bias=False) # no need for extra bias due to one in ln_f
@typecheck()
def forward(self, encoding):
x = self.ln_f(encoding)
logits = self.head(x)
return logits
@property
def input_types(self):
return {'encoding': NeuralType(('B', 'T', 'C'), EncodedRepresentation())}
@property
def output_types(self):
return {'logits': NeuralType(('B', 'T', 'C'), LogitsType())}
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