[Tune] Clarify which RunConfig is used when there are multiple places to specify it

[justinvyu]

Why are these changes needed?

This PR clarifies where RunConfig can be specified. Also, when multiple configs are specified in different locations (in the Tuner and Trainer), this PR also logs information about which RunConfig is actually used.

Context

• Scenario 1: RunConfig gets passed into both the Tuner and Trainer. The Tuner run config gets used, but this is not obvious/logged to the user.
• Scenario 2: User tries to pass RunConfig in through the param_space. This silently fails (aka it doesn't get used), and it shouldn't be allowed, since the RunConfig cannot be tuned.
• Scenario 3: Allowing a RunConfig in the Trainer when wrapping it with a Tuner makes it confusing whether or not this can be updated on restore, since it can be passed in again through the overwritten Trainable.

This PR logs something to the user for scenario 1, and disallows scenario 2.

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[krfricke]

Thanks!