Fix installation issues in virtual environment
Fixes installation issues in a normal python virtual environment when conda is also present on the same system. Also remove double negation.

Signed-off-by: Anand Joseph <anajoseph@nvidia.com>

# 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
