GPU binning and compression. (#3319)

* GPU binning and compression.

- binning and index compression are done inside the DeviceShard constructor
- in case of a DMatrix with multiple row batches, it is first converted into a single row batch

src/common/device_helpers.cuh

@@ -122,6 +122,14 @@ inline size_t AvailableMemory(int device_idx) {
   return device_free;
 }
 
+inline size_t TotalMemory(int device_idx) {
+  size_t device_free = 0;
+  size_t device_total = 0;
+  safe_cuda(cudaSetDevice(device_idx));
+  dh::safe_cuda(cudaMemGetInfo(&device_free, &device_total));
+  return device_total;
+}
+
 /**
  * \fn  inline int max_shared_memory(int device_idx)
  *
@@ -155,6 +163,12 @@ inline void CheckComputeCapability() {
   }
 }
 
+
+DEV_INLINE void AtomicOrByte(unsigned int* __restrict__ buffer, size_t ibyte, unsigned char b) {
+  atomicOr(&buffer[ibyte / sizeof(unsigned int)], (unsigned int)b << (ibyte % (sizeof(unsigned int)) * 8));
+}
+
+
 /*
  * Range iterator
  */