fix rect position bug.

* just a spell check

* some misspell

* nanodet python demo

* add clip

* fix clip wh

* remove nms package requirement

* fix model zoo download files

* fix yolov4.bin download split

* fix python/examples/shufflenetv2

* Update README.md

type in pip command to install requirements

* Update model_store.py

fix not working url. the models in the previous url were just stub.

* add python
* check elempack when convert ncnn.Mat to numpy.ndarray
fix build with NCNN_STRING=OFF