Make global placement min/max phi coef tunable

[jeffng-or]

Description

Create enh request to track discussion from staff meeting.

Suggested Solution

Steps:

1. Global_placement’s -min_phi_coef and -max_phi_coef are not exposed as flow variables, so we’d need to update global_place.tcl to expose them. Would we want to have the flow variables have a name that’s more user-friendly (e.g. MIN_PLACE_STEP_COEFF, MAX_PLACE_STEP_COEFF”
2. Add a check in global_place.tcl that ensures that MIN < MAX (covers if calling via ORFS)
3. Document the flow variables and mark them as tunable
4. Update AutoTuner _is_valid_config() to automatically flag if MIN > MAX (covers if AT chooses an invalid config and avoid running anything for an invalid config)

Additional Context

No response