PyTorch 1.6.0 update with native AMP (ultralytics#573)

requirements.txt

@@ -2,7 +2,7 @@
 Cython
 numpy>=1.18.5
 opencv-python
-torch>=1.5.1
+torch>=1.6
 matplotlib
 pillow
 tensorboard
@@ -12,9 +12,6 @@ scipy
 tqdm
 # pycocotools>=2.0
 
-# Nvidia Apex (optional) for mixed precision training --------------------------
-# git clone https://github.com/NVIDIA/apex && cd apex && pip install -v --no-cache-dir --global-option="--cpp_ext" --global-option="--cuda_ext" . --user && cd .. && rm -rf apex
-
 # Conda commands (in place of pip) ---------------------------------------------
 # conda update -yn base -c defaults conda
 # conda install -yc anaconda numpy opencv matplotlib tqdm pillow ipython