[export] streamline export and loading exported model

[fcdl94]

Improve the usability of exported model.
Now the pipeline to load an exported model requires to create a torch model and then export it using the correct runtime and cache=True.
However, it may be better an interface providing an option to export the model (only giving the model's format) and integrate the loading of exported models in the ModelManager.

[fcdl94]

From this

from focoos import ModelManager, FocoosHUB
from focoos.utils.vision import image_loader
hub = FocoosHUB(api_key="*************")
model = ModelManager.get("hub://xxxxx", hub=hub)
optimized_model = model.export(runtime_type=RuntimeType.ONNX_TRT16)

image = image_loader("./image.jpg")
detections = optimized_model(image)

To this

from focoos import ModelManager, FocoosHUB
from focoos.utils.vision import image_loader
hub = FocoosHUB(api_key="*************")
model = ModelManager.get("hub://xxxxx", hub=hub, runtime_type=RuntimeType.ONNX_TRT16)

image = image_loader("./image.jpg")
detections = model(image)