chore: make pod spec choice consistent

[carolinaecalderon]

Ticket

Description

We choose what pod spec to apply to a resource pool/experiment in two places, and two different ways. Let's try to harmonize this choice such that we will (more) consistently choose the same pod spec to define a pool and for experiments requested to run on it.

For example, if you're running a CPU cluster, you would intuitively want to define a cpuPodSpec for pods that experiments run on, and pods included in resource pools. Right now, GetAgents chooses which pod spec to use based on which one is defined AND device type (CPU vs CUDA). However, MergeExpConf chooses which pod spec to use based on how many slots an experiment wants (<=1 slot defaults to GPU pod spec). This makes it impossible to run experiments requiring more than 1 slot on a CPU cluster with a CPU podSpec defined.

Right now I'm merging this into my node selectors feature branch, but I can merge this into main after some rebasing. I will wait to merge https://github.com/determined-ai/determined/pull/9428 until this PR is merged.

Test Plan

Pass CircleCI + i'll write a test (in progress)

Checklist

• [ ] Changes have been manually QA'd
• [ ] User-facing API changes need the "User-facing API Change" label.
• [ ] Release notes should be added as a separate file under docs/release-notes/. See Release Note for details.
• [ ] Licenses should be included for new code which was copied and/or modified from any external code.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 51.36%. Comparing base (01c9b1d) to head (829c3f3).

Additional details and impacted files

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backend	34.25% <ø> (-9.70%)	:arrow_down:
harness	63.61% <ø> (-0.20%)	:arrow_down:
web	44.87% <ø> (ø)

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see 496 files with indirect coverage changes

[stoksc]

@carolinaecalderon what's up with this pr? just found it randomly.