Fix bias warmup LR init (#8356)

Per https://github.com/ultralytics/yolov5/issues/8352

train.py

@@ -335,7 +335,7 @@ def train(hyp, opt, device, callbacks):  # hyp is path/to/hyp.yaml or hyp dictio
                 accumulate = max(1, np.interp(ni, xi, [1, nbs / batch_size]).round())
                 for j, x in enumerate(optimizer.param_groups):
                     # bias lr falls from 0.1 to lr0, all other lrs rise from 0.0 to lr0
-                    x['lr'] = np.interp(ni, xi, [hyp['warmup_bias_lr'] if j == 2 else 0.0, x['initial_lr'] * lf(epoch)])
+                    x['lr'] = np.interp(ni, xi, [hyp['warmup_bias_lr'] if j == 0 else 0.0, x['initial_lr'] * lf(epoch)])
                     if 'momentum' in x:
                         x['momentum'] = np.interp(ni, xi, [hyp['warmup_momentum'], hyp['momentum']])