Bug Report: Incorrect filename for merging compressed datasets

[bhimrazy]

🐛 Bug

Bug reported in PR #190

Description:
The filename generation in functions.py does not account for the compression type. This results in incorrect filenames when compression == zstd.

# src/litdata/processing/functions.py

for chunk in input_dir_file_content["chunks"]:  # type: ignore
    assert isinstance(chunk, dict)
    old_filename = chunk["filename"]
    new_filename = f"chunk-0-{counter}.bin"