feat: add custom pypi indexes

[nichmor]

Add custom indexes for pixi like here: https://docs.astral.sh/uv/configuration/indexes/

A small example will be :

[project]
authors = ["nichmor <nmorkotilo@gmail.com>"]
channels = ["conda-forge"]
description = "Add a short description here"
name = "custom_pypi"
platforms = ["osx-arm64"]
version = "0.1.0"

[tasks]

[dependencies]
python = "*"


[pypi-dependencies]
tqdm ={ version = "*", index = "https://download.pytorch.org/whl/cu118" }