GPUTracking: give Metal its own Float16_t at the conversion site

[ktf]

Rather than trying to adapt Ort::Float16_t to work on Metal, we
simply alias it to Metal native type, which is bit-to-bit equivalent to the CUDA half
implementation and differs from the software one only in the sign of NaN.

[ktf]

@davidrohr @ChSonnabend as discussed over coffee.

[davidrohr]

Fine for now, let's clean it up more later.

[ktf]

Does not get tested right now. Merging.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 7f83cfa at 2026-09-25 12:58:

## sw/BUILD/O2-latest/log
Error: missing ; at end of rule at /sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/TPC/base/src/TPCBaseLinkDef.h:75:56
Error: missing ; at end of rule at /sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/TPC/base/src/TPCBaseLinkDef.h:83:135
Error: missing ; at end of rule at /sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/TPC/base/src/TPCBaseLinkDef.h:84:135
Error: missing ; at end of rule at /sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/ECal/base/src/ECalBaseLinkDef.h:19:48
Error: missing ; at end of rule at /sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/RICH/base/src/RICHBaseLinkDef.h:18:48
Error: missing ; at end of rule at /sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/MID/base/src/MI3BaseLinkDef.h:18:47


## sw/BUILD/O2-full-system-test-latest/log
command /sw/slc9_x86-64/O2/slc9_x86-64-slc9_x86-64-local3/prodtests/full-system-test/dpl-workflow.sh had nonzero exit code 1
[8892:internal-dpl-ccdb-backend]: [12:58:21][ERROR] CCDBDownloader CURL transfer error - Timeout was reached
[8892:internal-dpl-ccdb-backend]: [12:58:21][ERROR] CcdbDownloader finished transfer http://alice-ccdb.cern.ch/ZDC/Calib/EnergyCalib for 1550600800023 (agent_id: o2-ctf-reader-w/o2-ctf-reader-workflow@8892 target/Linux_x86_64 init/1790333835721 id/mesosci@pcapiserv07 session/fNhXbCG8) with http code: 0
[8892:internal-dpl-ccdb-backend]: [12:58:21][ERROR] File ZDC/Calib/EnergyCalib could not be retrieved. No more hosts to try.
[8892:internal-dpl-ccdb-backend]: [12:58:21][FATAL] Unable to find CCDB object ZDC/Calib/EnergyCalib/1550600800023
[8892:internal-dpl-ccdb-backend]: [12:58:22][ERROR] Exception while running: Fatal error. Rethrowing.
[8892:internal-dpl-ccdb-backend]: [12:58:22][FATAL] Unhandled o2::framework::runtime_error reached the top of main of o2-ctf-reader-workflow, device shutting down. Reason: Fatal error
[ERROR] Workflow crashed - PID 8892 (internal-dpl-ccdb-backend) did not exit correctly however it's not clear why. Exit code forced to 128.
[ERROR]  - Device internal-dpl-ccdb-backend: pid 8892 (exit 128)
[INFO]    - First error: [12:58:21][FATAL] Unable to find CCDB object ZDC/Calib/EnergyCalib/1550600800023
[ERROR] SEVERE: Device internal-dpl-ccdb-backend (8892) had at least one message above severity 7: Unable to find CCDB object ZDC/Calib/EnergyCalib/1550600800023


## sw/BUILD/o2checkcode-latest/log
--
========== List of errors found ==========
++ GRERR=0
++ grep -v clang-diagnostic-error error-log.txt
++ grep ' error:'
++ GRERR=1
++ [[ 1 == 0 ]]
++ mkdir -p /sw/INSTALLROOT/a0087847e6121fe228f7d1c25612a8cbefd41ef0/slc9_x86-64/o2checkcode/1.0-local3/etc/modulefiles
++ alibuild-generate-module
+ HAS_BIN=
+ HAS_LIB=
+ HAS_CMAKE=
+ HAS_ROOT=
+ '[' 0 '!=' 0 ']'
+ cat
+ printf 'if ![ is-loaded '\''BASE/1.0'\'' ] {\n module load BASE/1.0\n}'
+ echo 'BUILD_REQUIRES=CMake alibuild-recipe-tools defaults-release'
BUILD_REQUIRES=CMake alibuild-recipe-tools defaults-release
+ FULL_BUILD_REQUIRES='O2-customization CMake ninja date pytorch_cpuinfo Python-modules-list googlebenchmark defaults-release safe_int UUID ninja-fortran alibuild-recipe-tools Alice-GRID-Utils bz2 double-conversion json-c MPFR FairCMakeModules'
+ echo 'FULL_BUILD_REQUIRES=O2-customization CMake ninja date pytorch_cpuinfo Python-modules-list googlebenchmark defaults-release safe_int UUID ninja-fortran alibuild-recipe-tools Alice-GRID-Utils bz2 double-conversion json-c MPFR FairCMakeModules'
FULL_BUILD_REQUIRES=O2-customization CMake ninja date pytorch_cpuinfo Python-modules-list googlebenchmark defaults-release safe_int UUID ninja-fortran alibuild-recipe-tools Alice-GRID-Utils bz2 double-conversion json-c MPFR FairCMakeModules
++ env
++ cut -f1 -d=
++ grep -v '^DEFAULT_'
++ grep -v PKGREVISION
[0 more errors; see full log]

Full log here.