Extend array interface to handle ndarray. (#7434)

* Extend array interface to handle ndarray.

The `ArrayInterface` class is extended to support multi-dim array inputs. Previously this
class handles only 2-dim (vector is also matrix).  This PR specifies the expected
dimension at compile-time and the array interface can perform various checks automatically
for input data. Also, adapters like CSR are more rigorous about their input.  Lastly, row
vector and column vector are handled without intervention from the caller.

include/xgboost/data.h

@@ -11,12 +11,14 @@
 #include <dmlc/data.h>
 #include <dmlc/serializer.h>
 #include <xgboost/base.h>
-#include <xgboost/span.h>
 #include <xgboost/host_device_vector.h>
+#include <xgboost/linalg.h>
+#include <xgboost/span.h>
+#include <xgboost/string_view.h>
 
+#include <algorithm>
 #include <memory>
 #include <numeric>
-#include <algorithm>
 #include <string>
 #include <utility>
 #include <vector>
@@ -157,7 +159,7 @@ class MetaInfo {
    *
    *        Right now only 1 column is permitted.
    */
-  void SetInfo(const char* key, std::string const& interface_str);
+  void SetInfo(StringView key, std::string const& interface_str);
 
   void GetInfo(char const* key, bst_ulong* out_len, DataType dtype,
                const void** out_dptr) const;