[BUG] tiktoken is not optional dependency

[hynky1999]

Describe the bug

When installing from current branch, I am getting error that the tiktoken was not found

To Reproduce

Install from source and run any eval

2025-02-08 20:46:12.232490: E external/local_xla/xla/stream_executor/cuda/cuda_fft.cc:477] Unable to register cuFFT factory: Attempting to register factory for plugin cuFFT when one has already been registered
WARNING: All log messages before absl::InitializeLog() is called are written to STDERR
E0000 00:00:1739047572.447031    1808 cuda_dnn.cc:8310] Unable to register cuDNN factory: Attempting to register factory for plugin cuDNN when one has already been registered
E0000 00:00:1739047572.507275    1808 cuda_blas.cc:1418] Unable to register cuBLAS factory: Attempting to register factory for plugin cuBLAS when one has already been registered
2025-02-08 20:46:12.954291: I tensorflow/core/platform/cpu_feature_guard.cc:210] This TensorFlow binary is optimized to use available CPU instructions in performance-critical operations.
To enable the following instructions: AVX2 AVX512F FMA, in other operations, rebuild TensorFlow with the appropriate compiler flags.
Traceback (most recent call last):
  File "/content/./Math-Verify/evaluate_model.py", line 5, in <module>
    from lighteval.pipeline import ParallelismManager, Pipeline, PipelineParameters
  File "/usr/local/lib/python3.11/dist-packages/lighteval/pipeline.py", line 39, in <module>
    from lighteval.models.model_loader import TransformersModel, load_model
  File "/usr/local/lib/python3.11/dist-packages/lighteval/models/model_loader.py", line 32, in <module>
    from lighteval.models.endpoints.openai_model import OpenAIClient, OpenAIModelConfig
  File "/usr/local/lib/python3.11/dist-packages/lighteval/models/endpoints/openai_model.py", line 56, in <module>
    import tiktoken
ModuleNotFoundError: No module named 'tiktoken'

Expected behavior

Work without having tiktoken installed: