Squashed commit of the following:

commit 6552242
Author: Jiaming Yuan <[email protected]>
Date:   Fri Jun 16 05:17:52 2023 +0800

    Don't blame.

commit de4f71c
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 09:23:59 2023 +0800

    fix.

commit f3e39ac
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 09:11:10 2023 +0800

    read-only.

commit 914a186
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 09:06:59 2023 +0800

    read-only.

commit 6169fdc
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 08:59:18 2023 +0800

    cleanup.

commit 8b993ff
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 06:08:23 2023 +0800

    Forbid pointer to bool cast.

commit 076a788
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 02:55:33 2023 +0800

    fix.

commit b5b57a0
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 01:57:32 2023 +0800

    Improve test.

commit c8726c3
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 00:56:56 2023 +0800

    polishing.

commit 3d76acc
Merge: 22ae3f6 8cdbb87
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 00:54:19 2023 +0800

    Merge remote-tracking branch 'jiamingy/ext-mmap' into ext-mmap

commit 22ae3f6
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 00:54:05 2023 +0800

    reduce page number.

commit 8cdbb87
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 00:38:52 2023 +0800

    mingw

commit a4e11d3
Author: Jiaming Yuan <[email protected]>
Date:   Thu Jun 15 00:19:00 2023 +0800

    fix win leak

commit 94b8a0d
Author: Jiaming Yuan <[email protected]>
Date:   Wed Jun 14 18:31:34 2023 +0800

    Timer.

commit 9dd5812
Author: Jiaming Yuan <[email protected]>
Date:   Wed Jun 14 05:47:23 2023 +0800

    improve the tests.

commit 6a02601
Author: Jiaming Yuan <[email protected]>
Date:   Wed Jun 14 05:42:22 2023 +0800

    log time.

commit e88f561
Author: Jiaming Yuan <[email protected]>
Date:   Wed Jun 14 05:41:15 2023 +0800

    lint.

commit 788f2b6
Author: Jiaming Yuan <[email protected]>
Date:   Wed Jun 14 03:10:03 2023 +0800

    GPU compilation.

commit d3987e8
Author: Jiaming Yuan <[email protected]>
Date:   Wed Jun 14 02:57:42 2023 +0800

    cleanup.

commit 9bbecf5
Author: Jiaming Yuan <[email protected]>
Date:   Wed Jun 14 01:47:55 2023 +0800

    Cleanup.

commit c195db1
Author: Jiaming Yuan <[email protected]>
Date:   Wed Jun 14 01:25:40 2023 +0800

    Cleanup.

commit bcf4cdb
Author: Jiaming Yuan <[email protected]>
Date:   Wed Jun 14 01:14:03 2023 +0800

    Fix.

commit 925245c
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 22:11:48 2023 +0800

    debug.

commit 5baf5ca
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 20:34:13 2023 +0800

    Cleanup.

commit 58c0d99
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 20:29:27 2023 +0800

    Avoid padding the data.

commit 2660c66
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 19:06:59 2023 +0800

    Pad the file for windows.

commit 341c8fb
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 17:26:16 2023 +0800

    compile

commit 9068cc8
Merge: a61a079 4989269
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 17:10:52 2023 +0800

    Merge branch 'ext-mmap-win' into ext-mmap

commit 4989269
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 16:36:47 2023 +0800

    windows mmap

commit a61a079
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 05:20:16 2023 +0800

    doc.

commit a736125
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 04:03:12 2023 +0800

    lint.

commit 39ed218
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 03:40:09 2023 +0800

    comment.

commit 4521f04
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 03:35:22 2023 +0800

    use ctx.

commit 4b5d38f
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 02:33:18 2023 +0800

    GPU initialization.

commit 68b838d
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 02:15:29 2023 +0800

    remove in no sampling.

commit f383f76
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 02:13:33 2023 +0800

    remove page in uniform sampling.

commit 1b0dab2
Author: Jiaming Yuan <[email protected]>
Date:   Tue Jun 13 02:09:17 2023 +0800

    Remove page in grad-based sampling.

commit 9b5c686
Author: Jiaming Yuan <[email protected]>
Date:   Mon Jun 12 23:19:54 2023 +0800

    cleanup.

commit ed635d3
Author: Jiaming Yuan <[email protected]>
Date:   Mon Jun 12 23:17:17 2023 +0800

    rename.

commit 05ce49b
Author: Jiaming Yuan <[email protected]>
Date:   Mon Jun 12 23:12:30 2023 +0800

    Add test.

commit da00b6d
Author: Jiaming Yuan <[email protected]>
Date:   Mon Jun 12 21:59:21 2023 +0800

    lint.

commit 9ee1643
Author: Jiaming Yuan <[email protected]>
Date:   Mon Jun 12 21:56:55 2023 +0800

    Skip python test.

commit 117fb97
Author: Jiaming Yuan <[email protected]>
Date:   Mon Jun 12 21:45:27 2023 +0800

    Cleanup.

commit a6202d0
Author: Jiaming Yuan <[email protected]>
Date:   Mon Jun 12 02:46:18 2023 +0800

    cleanup.

commit ba358af
Author: Jiaming Yuan <[email protected]>
Date:   Mon Jun 12 02:45:50 2023 +0800

    Fix.

commit 1e0405e
Author: Jiaming Yuan <[email protected]>
Date:   Mon Jun 12 02:29:48 2023 +0800

    debug.

commit a04dc39
Author: Jiaming Yuan <[email protected]>
Date:   Sat Jun 10 00:30:18 2023 +0800

    macos.

commit 18c3544
Author: Jiaming Yuan <[email protected]>
Date:   Sat Jun 10 00:08:43 2023 +0800

    Cleanup.

commit 3832fd5
Author: Jiaming Yuan <[email protected]>
Date:   Fri Jun 9 23:02:47 2023 +0800

    cleanup.

commit 9ebd4ef
Author: Jiaming Yuan <[email protected]>
Date:   Fri Jun 9 22:51:24 2023 +0800

    abstract into a dmlc stream.

commit fa5d460
Author: Jiaming Yuan <[email protected]>
Date:   Fri Jun 9 12:42:47 2023 +0800

    cleanup.

commit 23a89b0
Author: Jiaming Yuan <[email protected]>
Date:   Fri Jun 9 12:25:08 2023 +0800

    reduce size.

commit b1573d4
Author: Jiaming Yuan <[email protected]>
Date:   Fri Jun 9 11:51:35 2023 +0800

    use mmap for external memory.

Author: trivialfis

src/collective/nccl_device_communicator.cu

@@ -70,7 +70,7 @@ NcclDeviceCommunicator::~NcclDeviceCommunicator() {
 
 namespace {
 ncclDataType_t GetNcclDataType(DataType const &data_type) {
-  ncclDataType_t result;
+  ncclDataType_t result{ncclInt8};
   switch (data_type) {
     case DataType::kInt8:
       result = ncclInt8;
@@ -108,7 +108,7 @@ bool IsBitwiseOp(Operation const &op) {
 }
 
 ncclRedOp_t GetNcclRedOp(Operation const &op) {
-  ncclRedOp_t result;
+  ncclRedOp_t result{ncclMax};
   switch (op) {
     case Operation::kMax:
       result = ncclMax;