Fixes for CI downloads (#14504)

* Fixes for data links

* Changed data.dmlc.ml -> data.mxnet.io
* Added retries for downloads
* Removed silent mode for explicit failure reporting

* Reworked download scripts

Author: lebeg

models/Inception/get.sh

@@ -18,5 +18,5 @@
 # under the License.
 
 
-wget -c http://data.dmlc.ml/mxnet/data/Inception.zip
+wget -c http://data.mxnet.io/mxnet/data/Inception.zip
 unzip Inception.zip