Make num_classes optional, in case of micro averaging
#10661
| Job | Run time |
|---|---|
| 1m 30s | |
| 1m 29s | |
| 1m 53s | |
| 1m 34s | |
| 2m 27s | |
| 3m 15s | |
| 2m 11s | |
| 2m 48s | |
| 3m 50s | |
| 3m 32s | |
| 3m 27s | |
| 3m 53s | |
| 1m 27s | |
| 33m 16s |