IndexError: index out of range in self

I currently work on the project of "Autism gene classifier " which is a binary-classification system .. I have a gene dataset which have columns gene-symbol and syndromic ( 0 and 1) ..

The Model i am using is GPT-2 and while i run my code on google colab i face the error of IndexError: index out of range in self

This is my code

# Install required libraries
!pip install torch
!pip install transformers
!pip install pandas
!pip install scikit-learn

# Import libraries
import pandas as pd
from sklearn.model_selection import train_test_split
from sklearn.linear_model import LogisticRegression
from sklearn.metrics import accuracy_score
from transformers import GPT2Tokenizer, GPT2Model
import torch

# Load your gene data (assuming you have a CSV file with 'gene_symbol' and 'syndromic' columns)
data = pd.read_csv('drive/MyDrive/Gene/sfari_genes.csv')

# Split the data into training and testing sets
train_data, test_data = train_test_split(data, test_size=0.2, random_state=42)

# Initialize GPT-2 tokenizer and model
tokenizer = GPT2Tokenizer.from_pretrained('gpt2')
model = GPT2Model.from_pretrained('gpt2')

# Add a new pad token
tokenizer.add_special_tokens({'pad_token': '[PAD]'})

# Tokenize and encode the gene symbols with a maximum length of 512 tokens
train_tokens = tokenizer(train_data['gene-symbol'].tolist(), padding=True, truncation=True, max_length=1024, return_tensors='pt')
test_tokens = tokenizer(test_data['gene-symbol'].tolist(), padding=True, truncation=True, max_length=1024, return_tensors='pt')

# Extract embeddings from GPT-2 model
model.eval()
with torch.no_grad():
train_embeddings = model(**train_tokens).last_hidden_state.mean(dim=1)
test_embeddings = model(**test_tokens).last_hidden_state.mean(dim=1)

# Flatten the embeddings to be used as input to logistic regression
train_embeddings = train_embeddings.view(train_embeddings.size(0), -1)
test_embeddings = test_embeddings.view(test_embeddings.size(0), -1)

# Train logistic regression classifier
clf = LogisticRegression(random_state=42)
clf.fit(train_embeddings, train_data['syndromic'])

# Evaluate logistic regression classifier
train_predictions = clf.predict(train_embeddings)
train_accuracy = accuracy_score(train_data['syndromic'], train_predictions)
print("Training accuracy:", train_accuracy)

when i try to extract embedding from GPT-2 Model this index error came...
i also tried to maximize length as 512 and 1024 .. it wont work for me...
How to resolve this error ...please solve the error