add cache_dir parameter

[BBC-Esq]

Currently, it's the program only allows you to use "huggingface," "local," or "custom" but none allow you to specify a directory where you want to download a file automatically using ```snapshot_download." This fixes that.

This pull request implements the following:

Here's a brief summary of the different options for handling model files in ChatTTS:

source="huggingface": Downloads models from HuggingFace repo to ~/.cache/huggingface

• or to cache_dir if specified

source="local": Downloads models from HuggingFace repo to current working directory

• or to cache_dir if specified

source="custom":

• No change

Example Usage:

from ChatTTS import Chat

# Option 1: Using huggingface source with custom cache directory
chat = Chat()
chat.load(
    source="huggingface",
    cache_dir="path/to/your/models",
    device="cuda"
)

# Option 2: Using local source with custom cache directory 
chat = Chat()
chat.load(
    source="local",
    cache_dir="path/to/your/models",
    device="cuda"
)

# Original behavior still works without cache_dir
chat = Chat()
chat.load(
    source="huggingface",  # Will use ~/.cache/huggingface
    device="cuda"
)