Fixes installation issues in a normal python virtual environment when conda is also present on the same system.
# What does this PR do ?

Fixes installation issues in a normal python virtual environment when conda is also present on the same system.

# Changelog 
- Check if "$CONDA_PREFIX" is set rather than check if "conda" is present. 
- Replace double negation "! -z" with "-n" when checking for NVIDIA_PYTORCH_VERSION
