Rename and extract Context. (#8528)

* Rename `GenericParameter` to `Context`.
* Rename header file to reflect the change.
* Rename all references.

src/data/iterative_dmatrix.h

@@ -86,7 +86,7 @@ class IterativeDMatrix : public DMatrix {
     LOG(FATAL) << "Slicing DMatrix is not supported for Quantile DMatrix.";
     return nullptr;
   }
-  DMatrix *SliceCol(std::size_t start, std::size_t size) override {
+  DMatrix *SliceCol(std::size_t, std::size_t) override {
     LOG(FATAL) << "Slicing DMatrix columns is not supported for Quantile DMatrix.";
     return nullptr;
   }

src/data/proxy_dmatrix.h

@@ -10,10 +10,10 @@
 #include <string>
 #include <utility>
 
-#include "xgboost/data.h"
-#include "xgboost/generic_parameters.h"
-#include "xgboost/c_api.h"
 #include "adapter.h"
+#include "xgboost/c_api.h"
+#include "xgboost/context.h"
+#include "xgboost/data.h"
 
 namespace xgboost {
 namespace data {
@@ -87,7 +87,7 @@ class DMatrixProxy : public DMatrix {
     LOG(FATAL) << "Slicing DMatrix is not supported for Proxy DMatrix.";
     return nullptr;
   }
-  DMatrix* SliceCol(std::size_t start, std::size_t size) override {
+  DMatrix* SliceCol(std::size_t, std::size_t) override {
     LOG(FATAL) << "Slicing DMatrix columns is not supported for Proxy DMatrix.";
     return nullptr;
   }

src/data/simple_dmatrix.cc

@@ -53,7 +53,7 @@ DMatrix* SimpleDMatrix::SliceCol(std::size_t start, std::size_t size) {
     auto& h_data = out_page.data.HostVector();
     auto& h_offset = out_page.offset.HostVector();
     size_t rptr{0};
-    for (auto i = 0; i < this->Info().num_row_; i++) {
+    for (bst_row_t i = 0; i < this->Info().num_row_; i++) {
       auto inst = batch[i];
       auto prev_size = h_data.size();
       std::copy_if(inst.begin(), inst.end(), std::back_inserter(h_data), [&](Entry e) {

src/data/sparse_page_dmatrix.h

@@ -107,7 +107,7 @@ class SparsePageDMatrix : public DMatrix {
     LOG(FATAL) << "Slicing DMatrix is not supported for external memory.";
     return nullptr;
   }
-  DMatrix *SliceCol(std::size_t start, std::size_t size) override {
+  DMatrix *SliceCol(std::size_t, std::size_t) override {
     LOG(FATAL) << "Slicing DMatrix columns is not supported for external memory.";
     return nullptr;
   }