[ColossalAI/examples/images/diffusion]: Solving the issue of incompatibility issue#3169

[NatalieC323]

Updated GitHub documentation:

1. requirements.txt content is incompatible with the version and cannot be downloaded. It is recommended to change the content opencv-python==4.6.0 to opencv-python==4.6.0.66 (otherwise an error will be reported saying that the corresponding version cannot be found). Similarly, torchmetrics==0.6 should be changed into torchmetrics==0.7 (otherwise it will raise an error of incompatibility)

2. The instructions 'pip install colossalai' and 'pip install xformer' given in Readme will forward to the latest version. To download the suitable version for this training, pip install colossalai==0.2.5, pip install xformer==0.12.0 is recommended instead

3. Cuda version installation guide is added to Readme. The recommended version is cuda11.6 to 11.8.