`gluon.Block.load_params` set the default `ctx` to cpu (#10160)
`gluon.Block.load_params` set the default `ctx` to cpu (#10160)

* `gluon.Block.load_params` set the default `ctx` to cpu

It's convenience to just have

```python
net.save_params('net.params')
net.load_params('net.params')
```

* Update block.py

jamesliu/mxnet,ForkedReposBak/mxnet,fullfanta/mxnet,szha/mxnet,yajiedesign/mxnet,ptrendx/mxnet,TuSimple/mxnet,TuSimple/mxnet,CodingCat/mxnet,ForkedReposBak/mxnet,yajiedesign/mxnet,eric-haibin-lin/mxnet,eric-haibin-lin/mxnet,CodingCat/mxnet,ptrendx/mxnet,precedenceguo/mxnet,TuSimple/mxnet,leezu/mxnet,jamesliu/mxnet,rahul003/mxnet,TuSimple/mxnet,luoyetx/mxnet,dmlc/mxnet,zhreshold/mxnet,larroy/mxnet,fullfanta/mxnet,zhreshold/mxnet,fullfanta/mxnet,reminisce/mxnet,mbaijal/incubator-mxnet,ForkedReposBak/mxnet,szha/mxnet,leezu/mxnet,leezu/mxnet,navrasio/mxnet,CodingCat/mxnet,DickJC123/mxnet,mbaijal/incubator-mxnet,CodingCat/mxnet,zhreshold/mxnet,navrasio/mxnet,luoyetx/mxnet,leezu/mxnet,precedenceguo/mxnet,sxjscience/mxnet,larroy/mxnet,jamesliu/mxnet,rahul003/mxnet,dmlc/mxnet,indhub/mxnet,DickJC123/mxnet,sxjscience/mxnet,ForkedReposBak/mxnet,jamesliu/mxnet,leezu/mxnet,tlby/mxnet,apache/incubator-mxnet,yajiedesign/mxnet,navrasio/mxnet,tlby/mxnet,indhub/mxnet,leezu/mxnet,rahul003/mxnet,precedenceguo/mxnet,precedenceguo/mxnet,ptrendx/mxnet,apache/incubator-mxnet,navrasio/mxnet,zhreshold/mxnet,rahul003/mxnet,dmlc/mxnet,sxjscience/mxnet,larroy/mxnet,szha/mxnet,reminisce/mxnet,tlby/mxnet,precedenceguo/mxnet,luoyetx/mxnet,navrasio/mxnet,eric-haibin-lin/mxnet,yajiedesign/mxnet,CodingCat/mxnet,mbaijal/incubator-mxnet,larroy/mxnet,TuSimple/mxnet,jamesliu/mxnet,leezu/mxnet,mbaijal/incubator-mxnet,DickJC123/mxnet,reminisce/mxnet,sxjscience/mxnet,fullfanta/mxnet,tlby/mxnet,ptrendx/mxnet,eric-haibin-lin/mxnet,ForkedReposBak/mxnet,CodingCat/mxnet,mbaijal/incubator-mxnet,tlby/mxnet,ForkedReposBak/mxnet,luoyetx/mxnet,jamesliu/mxnet,reminisce/mxnet,tlby/mxnet,szha/mxnet,reminisce/mxnet,eric-haibin-lin/mxnet,apache/incubator-mxnet,zhreshold/mxnet,CodingCat/mxnet,eric-haibin-lin/mxnet,tlby/mxnet,DickJC123/mxnet,zhreshold/mxnet,precedenceguo/mxnet,jamesliu/mxnet,ptrendx/mxnet,CodingCat/mxnet,mbaijal/incubator-mxnet,ForkedReposBak/mxnet,szha/mxnet,luoyetx/mxnet,precedenceguo/mxnet,yajiedesign/mxnet,CodingCat/mxnet,dmlc/mxnet,larroy/mxnet,luoyetx/mxnet,reminisce/mxnet,tlby/mxnet,larroy/mxnet,indhub/mxnet,fullfanta/mxnet,ForkedReposBak/mxnet,tlby/mxnet,indhub/mxnet,larroy/mxnet,fullfanta/mxnet,mbaijal/incubator-mxnet,dmlc/mxnet,yajiedesign/mxnet,szha/mxnet,reminisce/mxnet,yajiedesign/mxnet,mbaijal/incubator-mxnet,TuSimple/mxnet,ForkedReposBak/mxnet,ptrendx/mxnet,eric-haibin-lin/mxnet,ptrendx/mxnet,indhub/mxnet,jamesliu/mxnet,sxjscience/mxnet,zhreshold/mxnet,navrasio/mxnet,luoyetx/mxnet,dmlc/mxnet,larroy/mxnet,rahul003/mxnet,tlby/mxnet,rahul003/mxnet,larroy/mxnet,rahul003/mxnet,fullfanta/mxnet,TuSimple/mxnet,jamesliu/mxnet,dmlc/mxnet,precedenceguo/mxnet,ptrendx/mxnet,sxjscience/mxnet,luoyetx/mxnet,leezu/mxnet,navrasio/mxnet,sxjscience/mxnet,eric-haibin-lin/mxnet,sxjscience/mxnet,indhub/mxnet,zhreshold/mxnet,szha/mxnet,szha/mxnet,apache/incubator-mxnet,rahul003/mxnet,navrasio/mxnet,sxjscience/mxnet,indhub/mxnet,navrasio/mxnet,szha/mxnet,precedenceguo/mxnet,dmlc/mxnet,indhub/mxnet,dmlc/mxnet,reminisce/mxnet,ptrendx/mxnet,mbaijal/incubator-mxnet,ptrendx/mxnet,rahul003/mxnet,TuSimple/mxnet,fullfanta/mxnet,TuSimple/mxnet,reminisce/mxnet,yajiedesign/mxnet,eric-haibin-lin/mxnet,luoyetx/mxnet,TuSimple/mxnet,leezu/mxnet,zhreshold/mxnet,fullfanta/mxnet,indhub/mxnet,reminisce/mxnet,yajiedesign/mxnet