Make it more explicit about the rfactor flag propagation

[naoyam]

IterDomain::split has a default value of false for the rfactor flag, which means if an rfactor iter domain is split, it would generate non-rfactor output domains. I'm not sure if that's the right default behavior, so this PR removes the default value and make the behavior more explicit.

Nothing should be affected by this change, as long as all the tests and benchmarks don't fail.

[naoyam]

!build

[naoyam]

Failed cases:

NvFuserScheduler_Matmul_Manual/nvfuser_splitk_TT/M:1024/N:1024/K:50304/warps:8/stages:3/splitk_factor:2/smem_epilogue:0/manual_time for 1
00:00:10 terminate called after throwing an instance of 'nvfuser::nvfError'
00:00:10   what():   INTERNAL ASSERT FAILED at "/opt/pytorch/nvfuser/csrc/ir/internal_base_nodes.h":147, please report a bug with repro script to NVFuser at https://github.com/NVIDIA/Fuser/issues. Unexpected input iter domain. Input should not be an rfactor iter domain: rS196{32}rf
00:00:10 Exception raised from split at /opt/pytorch/nvfuser/csrc/ir/internal_base_nodes.h:147 (most recent call first):

0:00:06 -- LOG(2): Running NvFuserScheduler_Matmul_Manual/nvfuser_splitk_TT/M:136/N:184/K:175704/warps:4/stages:3/splitk_factor:2/smem_epilogue:0/manual_time for 1
00:00:06 terminate called after throwing an instance of 'nvfuser::nvfError'
00:00:06   what():   INTERNAL ASSERT FAILED at "/opt/pytorch/nvfuser/csrc/ir/internal_base_nodes.h":147, please report a bug with repro script to NVFuser at https://github.com/NVIDIA/Fuser/issues. Unexpected input iter domain. Input should not be an rfactor iter domain: rS196{32}rf
00:00:06 Exception raised from split at /opt/pytorch/nvfuser/csrc/ir/internal_base_nodes.h:147 (most recent call first):

[naoyam]

Actually, never mind. Rfactor should be only for between root and logical, so in normal cases the rfactor flag should be propagated to outputs.