Standardize default for model_cache_dir across repo

[klwetstone]

For densepose, there are multiple different default values specified for the model cache directory between the CLI and the python package

• In cli.py, says env variable ZAMBA_CACHE_DIR. It doesn't look like you can actually set it with this env variable

• In models.config.validate_model_cache_dir it's MODEL_CACHE_DIR

• In models.densepose.densepose_manager.DensePoseManager it's .zamba_cache

• [ ] use the same default cache dir throughout the repo - ZAMBA_CACHE_DIR in env likely makes more sense than MODEL_CACHE_DIR