Check And Remove Duplicates in chosen_metrics

[michaelmckinsey1]

@Yejashi Found a case where trying to add the same metric twice causes a bug.

Ex:

ncu_metrics = [
    "gpu__time_duration.sum",
    "gpu__time_duration.sum",
    "sm__throughput.avg.pct_of_peak_sustained_elapsed",
    "smsp__maximum_warps_avg_per_active_cycle",
    "smsp__maximum_warps_avg_per_active_cycle",
    "smsp__maximum_warps_avg_per_active_cycle",
]

is correctly handled now and prints a warning:

UserWarning: Removing duplicate metrics in chosen_metrics: ['gpu__time_duration.sum', 'smsp__maximum_warps_avg_per_active_cycle']

This is different from the functionality that checks if metrics in chosen_metrics are already in the performance data.

[ilumsden]

LGTM.

I have one comment about the performance of identifying the duplicates, but, IMO, it's not a big enough thing to block up this PR.

@pearce8 this PR is ready for your review.

[ilumsden]

This works, although it's not very efficient (collections.Counter is known for being pretty slow). That being said, we have other inefficiencies that we'll run into before this becomes a meaningful issue.