Replace symlink commands in Setup Guide Installation section with environment variable setup

[YipKo]

Changes:

In the Installation section of the Setup Guide, replace:

ln -sf $CONDA_PREFIX/lib/python3.10/site-packages/nvidia/*/include/* $CONDA_PREFIX/include/
ln -sf $CONDA_PREFIX/lib/python3.10/site-packages/nvidia/*/include/* $CONDA_PREFIX/include/python3.10

with:

export CPLUS_INCLUDE_PATH=$CONDA_PREFIX/lib/python3.10/site-packages/nvidia/cudnn/include:$CPLUS_INCLUDE_PATH
export C_INCLUDE_PATH=$CONDA_PREFIX/lib/python3.10/site-packages/nvidia/cudnn/include:$C_INCLUDE_PATH
export CPPFLAGS="-I$CONDA_PREFIX/lib/python3.10/site-packages/nvidia/cudnn/include $CPPFLAGS"

Benefit:

Avoids irreversible symlink operations, improving maintainability and environment isolation.

Tested Environment:

This change has been verified on two separate machines, both configured with the following environment:

torch==2.6.0+cu124
torchvision==0.21.0+cu124
diffusers==0.33.1
transformers==4.51.3
transformer_engine==1.13.0
megatron.core==0.10.0
flash_attn==2.6.3