Fix compiler warnings. (#7974)

- Remove unused parameters. There are still many warnings that are not yet
addressed. Currently, the warnings in dmlc-core dominate the error log.
- Remove `distributed` parameter from metric.
- Fixes some warnings about signed comparison.

include/xgboost/gbm.h

@@ -68,8 +68,8 @@ class GradientBooster : public Model, public Configurable {
    * \param layer_end   End of booster layer. 0 means do not limit trees.
    * \param out         Output gradient booster
    */
-  virtual void Slice(int32_t layer_begin, int32_t layer_end, int32_t step,
-                     GradientBooster *out, bool* out_of_bound) const {
+  virtual void Slice(int32_t /*layer_begin*/, int32_t /*layer_end*/, int32_t /*step*/,
+                     GradientBooster* /*out*/, bool* /*out_of_bound*/) const {
     LOG(FATAL) << "Slice is not supported by current booster.";
   }
   /*!

include/xgboost/json_io.h

@@ -89,7 +89,7 @@ class JsonReader {
     } else if (got == 0) {
       msg += "\\0\"";
     } else {
-      msg += (got <= 127 ? std::string{got} : std::to_string(got)) + " \"";  // NOLINT
+      msg += (got <= static_cast<char>(127) ? std::string{got} : std::to_string(got)) + " \"";
     }
     Error(msg);
   }

include/xgboost/linalg.h

@@ -317,7 +317,8 @@ class TensorView {
   }
 
   template <size_t old_dim, size_t new_dim, int32_t D, typename Index>
-  LINALG_HD size_t MakeSliceDim(size_t new_shape[D], size_t new_stride[D], Index i) const {
+  LINALG_HD size_t MakeSliceDim(DMLC_ATTRIBUTE_UNUSED size_t new_shape[D],
+                                DMLC_ATTRIBUTE_UNUSED size_t new_stride[D], Index i) const {
     static_assert(old_dim < kDim, "");
     return stride_[old_dim] * i;
   }

include/xgboost/metric.h

@@ -57,12 +57,8 @@ class Metric : public Configurable {
    * \brief evaluate a specific metric
    * \param preds prediction
    * \param info information, including label etc.
-   * \param distributed whether a call to Allreduce is needed to gather
-   *        the average statistics across all the node,
-   *        this is only supported by some metrics
    */
-  virtual double Eval(const HostDeviceVector<bst_float> &preds,
-                      const MetaInfo &info, bool distributed) = 0;
+  virtual double Eval(const HostDeviceVector<bst_float>& preds, const MetaInfo& info) = 0;
   /*! \return name of metric */
   virtual const char* Name() const = 0;
   /*! \brief virtual destructor */

include/xgboost/objective.h

@@ -103,8 +103,10 @@ class ObjFunction : public Configurable {
    * \param prediction Model prediction after transformation.
    * \param p_tree Tree that needs to be updated.
    */
-  virtual void UpdateTreeLeaf(HostDeviceVector<bst_node_t> const& position, MetaInfo const& info,
-                              HostDeviceVector<float> const& prediction, RegTree* p_tree) const {}
+  virtual void UpdateTreeLeaf(HostDeviceVector<bst_node_t> const& /*position*/,
+                              MetaInfo const& /*info*/,
+                              HostDeviceVector<float> const& /*prediction*/,
+                              RegTree* /*p_tree*/) const {}
 
   /*!
    * \brief Create an objective function according to name.