Add docs for get_installed_torch_platform and get_device in utils

Author: cmdr2

API.md

@@ -23,13 +23,34 @@ gpus = get_gpus()  # Returns a list of `torchruntime.device_db.GPU` instances co
 
 **Important:** This API could break in the future, so if you're writing a program using this, please open a new Issue on this repo and let me know what you're trying to do.
 
-## Get torch platform
+## Get torch platform (given any GPU)
 This will return the recommended torch platform to use for the PC. It will analyze the GPUs and OS on the PC, and suggest the most-performant version of torch for that.
 
+*Note: this is different from the installed torch platfrom (see the next paragraph)!*
+
 E.g. `cu124` or `rocm6.1` or `directml` or `ipex` or `xpu` or `cpu`.
 
 ```py
 from torchruntime.platform_detection import get_torch_platform
 
 torch_platform = get_torch_platform(gpus)  # use `torchruntime.device_db.get_gpus()` to get a list of recognized GPUs
 ```
+
+## Get installed torch platform
+This will return the installed torch platform, if any. E.g. "cuda", "mps", "cpu" etc.
+
+```py
+from torchruntime.utils import get_installed_torch_platform
+
+torch_platform = get_installed_torch_platform()[0]
+```
+
+## Get torch device
+This will return an instance of `torch.device` for the given device index, from the currently installed torch platform.
+
+```py
+from torchruntime.utils import get_device
+
+device1 = get_device(0)
+device2 = get_device("cuda:1")
+```