Set explicit GPU defaults in ResourcesSpec and improve default GPU vendor selection

[peterschmidt85]

Motivation

Previously, ResourcesSpec.gpu defaulted to None, making GPU the only resource without an explicit default (CPU defaults to 2.., memory to 8GB.., disk to 100GB..). This caused the GPU field to be hidden in the CLI plan output. Additionally, the NVIDIA vendor default was silently applied deep in the validation code regardless of whether the user set a custom image.

Changes

• Set ResourcesSpec.gpu default to GPUSpec(count=0..) instead of None, aligning GPU with other resource defaults
• Default GPU vendor to nvidia only when using the default CUDA image (no custom image or docker). With a custom image, any vendor is allowed
• Server-side vendor inference via set_gpu_vendor_default() with full backward compatibility across old/new CLI and server combinations

Before / After

# Before (no GPU, no image)
Resources  cpu=2.. mem=8GB.. disk=100GB..

# After
Resources  cpu=2.. mem=8GB.. disk=100GB.. gpu=nvidia:0..

# Before (gpu: 1, no image)
Resources  cpu=2.. mem=8GB.. disk=100GB.. gpu:1

# After
Resources  cpu=2.. mem=8GB.. disk=100GB.. gpu=nvidia:1

# Before (gpu: 1, custom image)
Resources  cpu=2.. mem=8GB.. disk=100GB.. gpu:1

# After - any vendor allowed since user provides their own image
Resources  cpu=2.. mem=8GB.. disk=100GB.. gpu=1

Backward compatibility

Tested across all CLI/server combinations (new CLI 0.21-dev, old CLI 0.20.9, new server, old server on sky.dstack.ai). GCP backend used for tests (has both NVIDIA and TPU offers).

dstack apply

Row	Config	CLI	Server	Display	TPU included?
1	No GPU, no image	New	New	gpu=nvidia:0..	No
2	No GPU, custom image	New	New	gpu=0..	Yes
3	No GPU, docker=true	New	New	gpu=0..	Yes
4	gpu: 1, no image	New	New	gpu=nvidia:1	No
5	gpu: 1, custom image	New	New	gpu=1	Yes
6	gpu: 1, docker=true	New	New	gpu=1	Yes
7	gpu: A100	New	New	gpu=A100:1..	No
8	gpu: MI300X, no image	New	New	Error: image required	N/A
9	gpu: MI300X, image	New	New	gpu=MI300X:1..	No
10	gpu: 1..4, no image	New	New	gpu=nvidia:1..4	No
11	gpu: nvidia:1	New	New	gpu=nvidia:1	No
12	gpu: amd:1, image	New	New	gpu=amd:1	No
13	No GPU, no image	Old 0.20.9	New	(no gpu shown)	Yes
14	gpu: 1, no image	Old 0.20.9	New	gpu:1	No
15	gpu: 1, custom image	Old 0.20.9	New	gpu:1	No
16	gpu: A100	Old 0.20.9	New	A100:1..	No
17	No GPU, no image	New	Old (sky)	gpu=0..	Yes
18	gpu: 1, no image	New	Old (sky)	gpu=1	No
19	gpu: 1, custom image	New	Old (sky)	gpu=1	No

dstack offer

Row	Config	CLI	Server	Display	TPU included?
20	--gpu 1	New	New	gpu=1	Yes
21	--gpu nvidia:1	New	New	gpu=nvidia:1	No
22	--gpu tpu:1	New	New	gpu=google:1	Yes (TPU only)
23	--gpu 1	New	Old (sky)	gpu=1	Yes
24	default	Old 0.20.9	New	(no gpu shown)	Yes
25	--gpu 1	Old 0.20.9	New	gpu:1	No

Key observations

• No regressions in any old CLI + old server combination
• Row 15: old CLI always sets vendor=nvidia regardless of image (pre-existing behavior)
• Row 19: old server always infers nvidia regardless of image (pre-existing behavior)

[peterschmidt85]

TODOs

Docs updates:

• Update gpu property description in concepts pages (dev-environments.md, tasks.md, services.md) and protips.md to reflect:
  • No gpu specified → defaults to 0.. (any vendor)
  • No count specified → defaults to 1..
  • Vendor defaults to nvidia only when no custom image is set; with custom image, any vendor is allowed
• Update reference schema docs accordingly

Future scope (outside this PR):

• Consider making TPU an explicit opt-in for GCP backend, since TPU requires specific setup and silently offering TPU instances may lead to unexpected behavior