Improved model+EMA checkpointing (#2292)

* Enhanced model+EMA checkpointing

* update

* bug fix

* bug fix 2

* always save optimizer

* ema half

* remove model.float()

* model half

* carry ema/model in fp32

* rm model.float()

* both to float always

* cleanup

* cleanup

utils/general.py

@@ -484,8 +484,8 @@ def non_max_suppression(prediction, conf_thres=0.25, iou_thres=0.45, classes=Non
 def strip_optimizer(f='weights/best.pt', s=''):  # from utils.general import *; strip_optimizer()
     # Strip optimizer from 'f' to finalize training, optionally save as 's'
     x = torch.load(f, map_location=torch.device('cpu'))
-    for key in 'optimizer', 'training_results', 'wandb_id':
-        x[key] = None
+    for k in 'optimizer', 'training_results', 'wandb_id', 'ema':  # keys
+        x[k] = None
     x['epoch'] = -1
     x['model'].half()  # to FP16
     for p in x['model'].parameters():