Add identity matrix function

[lowener]

This PR adds the identity matrix function eye (will be used in LOBPCG) and fixes a check on the layout for norm function.

[cjnolet]

Changes look great, @lowener. Thanks!

[cjnolet]

@lowener seeing some failures in MegeLabelsTest:

2023-06-05T17:31:20.0570208Z [ RUN      ] MergeLabelsTests/MergeLabelsTestL.Result/5
2023-06-05T17:31:20.0570656Z unknown file: Failure
2023-06-05T17:31:20.0572019Z C++ exception with description "CUDA error encountered at: file=/opt/conda/conda-bld/work/cpp/include/raft/util/cudart_utils.hpp line=183: call='cudaMemcpyAsync(dst, src, len * sizeof(Type), cudaMemcpyDefault, stream)', Reason=cudaErrorInvalidValue:invalid argument
2023-06-05T17:31:20.0572815Z Obtained 13 stack frames
2023-06-05T17:31:20.0573315Z #0 in /opt/conda/envs/test/bin/gtests/libraft/LABEL_TEST(+0xe29c) [0x564eac26c29c]
2023-06-05T17:31:20.0573893Z #1 in /opt/conda/envs/test/bin/gtests/libraft/LABEL_TEST(+0xea95) [0x564eac26ca95]
2023-06-05T17:31:20.0574470Z #2 in /opt/conda/envs/test/bin/gtests/libraft/LABEL_TEST(+0x1f60c) [0x564eac27d60c]
2023-06-05T17:31:20.0575043Z #3 in /opt/conda/envs/test/bin/gtests/libraft/LABEL_TEST(+0x199c0) [0x564eac2779c0]
2023-06-05T17:31:20.0575912Z #4 in /opt/conda/envs/test/bin/gtests/libraft/../../../lib/libgtest.so.1.13.0(_ZN7testing8internal35HandleExceptionsInMethodIfSupportedINS_4TestEvEET0_PT_MS4_FS3_vEPKc+0x4e) [0x7fc6c975cb5e]
2023-06-05T17:31:20.0576861Z #5 in /opt/conda/envs/test/bin/gtests/libraft/../../../lib/libgtest.so.1.13.0(_ZN7testing4Test3RunEv+0xe1) [0x7fc6c975ce31]
2023-06-05T17:31:20.0577602Z #6 in /opt/conda/envs/test/bin/gtests/libraft/../../../lib/libgtest.so.1.13.0(_ZN7testing8TestInfo3RunEv+0x19f) [0x7fc6c975d1ff]
2023-06-05T17:31:20.0578353Z #7 in /opt/conda/envs/test/bin/gtests/libraft/../../../lib/libgtest.so.1.13.0(_ZN7testing9TestSuite3RunEv+0xf4) [0x7fc6c975d554]
2023-06-05T17:31:20.0579208Z #8 in /opt/conda/envs/test/bin/gtests/libraft/../../../lib/libgtest.so.1.13.0(_ZN7testing8internal12UnitTestImpl11RunAllTestsEv+0x3bb) [0x7fc6c9765fdb]
2023-06-05T17:31:20.0580040Z #9 in /opt/conda/envs/test/bin/gtests/libraft/../../../lib/libgtest.so.1.13.0(_ZN7testing8UnitTest3RunEv+0x86) [0x7fc6c975d766]
2023-06-05T17:31:20.0580892Z #10 in /opt/conda/envs/test/bin/gtests/libraft/../../../lib/libgtest_main.so.1.13.0(main+0x3f) [0x7fc6c9a2706f]
2023-06-05T17:31:20.0581661Z #11 in /usr/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf3) [0x7fc6c9332083]
2023-06-05T17:31:20.0582238Z #12 in /opt/conda/envs/test/bin/gtests/libraft/LABEL_TEST(+0xac5e) [0x564eac268c5e]
2023-06-05T17:31:20.0582711Z " thrown in the test body.
2023-06-05T17:31:20.0584177Z [  FAILED  ] MergeLabelsTests/MergeLabelsTestL.Result/5, where GetParam() = 104-byte object <08-00 00-00 00-00 00-00 40-1F CB-AD 4E-56 00-00 80-1F CB-AD 4E-56 00-00 80-1F CB-AD 4E-56 00-00 A0-23 CB-AD 4E-56 00-00 E0-23 CB-AD 4E-56 00-00 E0-23 CB-AD 4E-56 00-00 B0-EB CA-AD 4E-56 00-00 B8-EB CA-AD 4E-56 00-00 B8-EB CA-AD 4E-56 00-00 A0-25 CB-AD 4E-56 00-00 E0-25 CB-AD 4E-56 00-00 E0-25 CB-AD 4E-56 00-00> (0 ms)
2023-06-05T17:31:20.0585454Z [----------] 6 tests from MergeLabelsTests/MergeLabelsTestL (6 ms total)

[lowener]

That's weird, merge_labels is just using raft::linalg::range (calling raft::linalg::map) which I didn't touch.
No file from raft/matrix is even included.
I tried to replicate it locally but I'm not seeing these failures.
I will merge the latest branch to re-trigger the CI.

[cjnolet]

@lowener I realized last night that these tests also seem to be failing in the nightlies. I think this might be a case where a recent PR was merged before being officially "updated" in GitHub (which is no longer required).

I'm looking into this.

[cjnolet]

/merge