[R] Add missing DMatrix functions (#9929)

* `XGDMatrixGetQuantileCut`
* `XGDMatrixNumNonMissing`
* `XGDMatrixGetDataAsCSR`

---------

Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>

R-package/man/xgb.get.DMatrix.data.Rd

@@ -0,0 +1,19 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/xgb.DMatrix.R
+\name{xgb.get.DMatrix.data}
+\alias{xgb.get.DMatrix.data}
+\title{Get DMatrix Data}
+\usage{
+xgb.get.DMatrix.data(dmat)
+}
+\arguments{
+\item{dmat}{An \code{xgb.DMatrix} object, as returned by \link{xgb.DMatrix}.}
+}
+\value{
+The data held in the DMatrix, as a sparse CSR matrix (class \code{dgRMatrix}
+from package \code{Matrix}). If it had feature names, these will be added as column names
+in the output.
+}
+\description{
+Get DMatrix Data
+}

R-package/man/xgb.get.DMatrix.num.non.missing.Rd

@@ -0,0 +1,17 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/xgb.DMatrix.R
+\name{xgb.get.DMatrix.num.non.missing}
+\alias{xgb.get.DMatrix.num.non.missing}
+\title{Get Number of Non-Missing Entries in DMatrix}
+\usage{
+xgb.get.DMatrix.num.non.missing(dmat)
+}
+\arguments{
+\item{dmat}{An \code{xgb.DMatrix} object, as returned by \link{xgb.DMatrix}.}
+}
+\value{
+The number of non-missing entries in the DMatrix
+}
+\description{
+Get Number of Non-Missing Entries in DMatrix
+}

R-package/man/xgb.get.DMatrix.qcut.Rd

@@ -0,0 +1,58 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/xgb.DMatrix.R
+\name{xgb.get.DMatrix.qcut}
+\alias{xgb.get.DMatrix.qcut}
+\title{Get Quantile Cuts from DMatrix}
+\usage{
+xgb.get.DMatrix.qcut(dmat, output = c("list", "arrays"))
+}
+\arguments{
+\item{dmat}{An \code{xgb.DMatrix} object, as returned by \link{xgb.DMatrix}.}
+
+\item{output}{Output format for the quantile cuts. Possible options are:\itemize{
+\item \code{"list"} will return the output as a list with one entry per column, where
+each column will have a numeric vector with the cuts. The list will be named if
+\code{dmat} has column names assigned to it.
+\item \code{"arrays"} will return a list with entries \code{indptr} (base-0 indexing) and
+\code{data}. Here, the cuts for column 'i' are obtained by slicing 'data' from entries
+\code{indptr[i]+1} to \code{indptr[i+1]}.
+}}
+}
+\value{
+The quantile cuts, in the format specified by parameter \code{output}.
+}
+\description{
+Get the quantile cuts (a.k.a. borders) from an \code{xgb.DMatrix}
+that has been quantized for the histogram method (\code{tree_method="hist"}).
+
+These cuts are used in order to assign observations to bins - i.e. these are ordered
+boundaries which are used to determine assignment condition \verb{border_low < x < border_high}.
+As such, the first and last bin will be outside of the range of the data, so as to include
+all of the observations there.
+
+If a given column has 'n' bins, then there will be 'n+1' cuts / borders for that column,
+which will be output in sorted order from lowest to highest.
+
+Different columns can have different numbers of bins according to their range.
+}
+\examples{
+library(xgboost)
+data(mtcars)
+y <- mtcars$mpg
+x <- as.matrix(mtcars[, -1])
+dm <- xgb.DMatrix(x, label = y, nthread = 1)
+
+# DMatrix is not quantized right away, but will be once a hist model is generated
+model <- xgb.train(
+  data = dm,
+  params = list(
+    tree_method = "hist",
+    max_bin = 8,
+    nthread = 1
+  ),
+  nrounds = 3
+)
+
+# Now can get the quantile cuts
+xgb.get.DMatrix.qcut(dm)
+}