[doc] Update installation instructions to include (mamba) pytorch-gpu installation tips

doc/install.rst

@@ -45,6 +45,18 @@ We support two installation modes, through pip_, or mamba_ (conda).
 
       mamba install -c https://www.idiap.ch/software/biosignal/conda/label/beta -c conda-forge ptbench
 
+   .. tip::
+
+      To force-install Nvidia GPU support on Linux machines, execute:
+
+      .. code:: sh
+
+         $ mamba install pytorch-gpu
+
+         # or, to force the Nvidia CUDA version (environments w/o Nvidia setup):
+
+         $ CONDA_OVERRIDE_CUDA=11.2 mamba install 'pytorch-gpu=*=cuda112*'
+
 
 .. _ptbench.setup: