don't initialize cuda at import time

[technillogue]

Description

Currently importing torch_tensorrt calls _cuda_init, which can fail, preventing torch_tensorrt being imported in environments without a (working) GPU. This makes it slow to import torch_tensorrt and isn't necessary. This change switches to determining the current devices when TensorRTCompileSpec is called rather than when it's defined.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code follows the style guidelines of this project (You can use the linters)
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas and hacks
• [x] I have made corresponding changes to the documentation
• [x] I have added tests to verify my fix or my feature
• [x] New and existing unit tests pass locally with my changes
• [ ] I have added the relevant labels to my PR in so that relevant reviewers are notified

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[technillogue]

I've made the change