Enhance inplace prediction. (#6653)

* Accept array interface for csr and array.
* Accept an optional proxy dmatrix for metainfo.

This constructs an explicit `_ProxyDMatrix` type in Python.

* Remove unused doc.
* Add strict output.

tests/python/test_with_shap.py

@@ -1,6 +1,5 @@
 import numpy as np
 import xgboost as xgb
-import testing as tm
 import pytest
 
 try: