[Target] Refine equality check on TargetKind instances

[wrongtest-intellif]

The change propose to ensure the TargetKind instances to be singleton on serialization.

We encounter the issue when we try to establish distributed tools to accelerate the lowering. Though TargetKind are generally singleton instances for each registered kind, the workload serialization would also create new TargetKind instances (which mostly lay in kTarget func attr).

One simple test workflow is like below:

target = Target("llvm")
mod = tvm.IRModule({"main": func})

# in practice, module dict is collected from many lowerred workloads, with target deserialized.
target2 = tvm.ir.load_json(tvm.ir.save_json(target))
lib = tvm.build({target2: mod}, target_host=target)
lib["func"]()  # oops, no "func" found in the host module.

[wrongtest-intellif]

@tqchen Hi, could you kindly take a look at the change? CI / Windows (pull_request) seems to be broken but I can't find why.

[tqchen]

thank you @wrongtest-intellif !