Swap byte-order in binary serializer to support big-endian arch (#5813)

* fixed some endian issues

* Use dmlc::ByteSwap() to simplify code

* Fix lint check

* [CI] Add test for s390x

* Download latest CMake on s390x

* Fix a bug in my code

* Save magic number in dmatrix with byteswap on big-endian machine

* Save version in binary with byteswap on big-endian machine

* Load scalar with byteswap in MetaInfo

* Add a debugging message

* Handle arrays correctly when byteswapping

* EOF can also be 255

* Handle magic number in MetaInfo carefully

* Skip Tree.Load test for big-endian, since the test manually builds little-endian binary model

* Handle missing packages in Python tests

* Don't use boto3 in model compatibility tests

* Add s390 Docker file for local testing

* Add model compatibility tests

* Add R compatibility test

* Revert "Add R compatibility test"

This reverts commit c2d2bdcb7dbae133cbb927fcd20f7e83ee2b18a8.

Co-authored-by: Qi Zhang <q.zhang@ibm.com>
Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

python-package/xgboost/core.py

@@ -1465,8 +1465,12 @@ class Booster(object):
                            ctypes.c_uint(iteration_range[1]))
 
         # once caching is supported, we can pass id(data) as cache id.
-        if isinstance(data, DataFrame):
-            data = data.values
+        try:
+            import pandas as pd
+            if isinstance(data, pd.DataFrame):
+                data = data.values
+        except ImportError:
+            pass
         if isinstance(data, np.ndarray):
             assert data.flags.c_contiguous
             arr = np.array(data.reshape(data.size), copy=False,