Fix Oumi launcher to be able to run on RunPod and Lambda

[wizeng23]

Description

Two bug fixes were made to enable running on RunPod and Lambda:

1. Change the default value of disk_tier from "medium" to None. This is because only AWS, Azure, GCP, and OCI support low/medium/high disk tiers, and the rest only support "best'.
   • This should still default to medium in SkyPilot, for clouds that support it I assume: https://github.com/skypilot-org/skypilot/blob/8ee8230c5519d42d0ac8fb9203f01a8950377f92/sky/resources.py#L140.
   • IMO we can also set the default value to "best", as disk cost is likely minimal compared to GPU cost, and using higher tiers makes disk I/O less likely to bottleneck.
2. Only set autostop if the cloud supports it. RunPod and Lambda both don't support it. Referencing this code: https://github.com/skypilot-org/skypilot/blob/aa3c387f04fbdd4468751b7d66fcb381bd3449dc/sky/clouds/cloud.py#L592

Tested that launching on RunPod works with these fixes.

Related issues

Fixes #1458
Fixes #1459
Fixes #1461

Before submitting

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[nikg4]

please check if it still works on GCP

[wizeng23]

Confirmed