Add Model Download Script and Update Configs (#1)

[ppmzhang2]

Added model download script and related updates:

• New .gitignore file
• Environment setup: updated requirements.txt with additional packages, and add an optional conda environment file (conda-nix-vc-py310.yaml).
• Configuration updates: all models now reside in the checkpoints/ directory, and paths in config_dit_mel_seed.yml, hifigan.yml and seed_vc_test.py are updated accordingly.
• A script download_models.py is added to automate the download of required models from Hugging Face Hub.

[Plachtaa]

Hi there, thanks for your effort and contribution! The code looks nice and will merge it after some local testing on my end😊