docs: add Google Colab setup and troubleshooting section (#2025)

### Summary
Add a Colab-specific setup and troubleshooting section to README to help
users run MONAI tutorials reliably on Google Colab.

### Changes
- Document explicit installation of PyTorch 2.6.0 (CUDA 12.4) and MONAI
1.5 on Colab
- Add troubleshooting notes (torchaudio mismatch, filelock conflict,
num_workers warning)
- Include a quick smoke test command to verify the setup

### Motivation
Many users start with Colab and often hit dependency conflicts. This
section improves first-time success and reproducibility.

---------

Signed-off-by: minsu <minsu.kim08@gmail.com>
Signed-off-by: Minsu Kim <60283244+minsuking@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: 3 people

README.md

@@ -41,6 +41,50 @@ Running:
 
 in a cell will verify this has worked and show you what kind of hardware you have access to.
 
+#### Google Colab Setup (CUDA 12.x, PyTorch 2.6, MONAI 1.5)
+
+In Google Colab, the default environment may cause version conflicts with MONAI.
+To ensure compatibility, install PyTorch and MONAI explicitly as follows:
+
+# Install PyTorch 2.6.0 with CUDA 12.4
+pip install --index-url https://download.pytorch.org/whl/cu124 \
+  torch==2.6.0 torchvision==0.21.0 torchaudio==2.6.0
+
+# Install MONAI and common dependencies
+pip install "monai[all]" nibabel pydicom ipywidgets==8.1.2
+
+
+### Known issues and fixes
+
+- Torchaudio mismatch
+  Colab may come with torchaudio 2.8.0, which is incompatible with torch 2.6.0.
+  Installing the versions above resolves this issue.
+
+- filelock conflicts with nni
+  Some preinstalled packages (such as pytensor with newer filelock) may conflict.
+  Use the following commands to fix:
+
+  pip uninstall -y pytensor
+  pip install -U filelock
+
+- Too many workers warning
+  Colab has limited CPU resources, and high num_workers settings may freeze execution.
+  It is recommended to use --num_workers=2 when running tutorials and adjust the `num_workers` parameters where it is used in notebooks (eg. for data loaders).
+
+
+### Quick smoke test
+
+After installation, verify the environment by running:
+
+git clone https://github.com/Project-MONAI/tutorials.git
+cd tutorials/3d_segmentation/torch
+python -u unet_training_array.py --max_epochs 2 --batch_size 1 --num_workers 2
+
+If the logs show decreasing training loss and a Dice score, the setup is correct.
+
+**Note:** In most cases, users can run MONAI tutorials directly in Colab notebooks without additional installation.
+The steps above are mainly for resolving dependency conflicts when installing extra packages.
+
 #### Data
 
 Some notebooks will require additional data.