`register_initializer` should create an unique name to resolve name collisions

[titaiwangms]

The feature was initially proposed here: microsoft/onnxscript#2680 (comment), where we saw a silent name collision leading to problematic optimized model.

We should firstly refactor rewrite rules to use the official initializer registration API to avoid silent errors:
https://github.com/microsoft/onnxscript/blob/478acf741471a4764d507d52a4ca6b544a63fc86/onnxscript/rewriter/_rewrite_rule.py#L685-L691

Also, instead of throwing errors, register_initializer can solve this automatically:

ir-py/src/onnx_ir/_core.py

Lines 2727 to 2743 in 926397c

	def register_initializer(self, value: Value) -> None:
	"""Register an initializer to the graph.
	This is a convenience method to register an initializer to the graph with
	checks.
	Args:
	value: The :class:`Value` to register as an initializer of the graph.
	It must have its ``.const_value`` set.
	Raises:
	ValueError: If a value of the same name that is not this value
	is already registered.
	ValueError: If the value does not have a name.
	ValueError: If the initializer is produced by a node.
	ValueError: If the value does not have its ``.const_value`` set.
	"""

cc @justinchuby @xadupre @gramalingam more thoughts

[justinchuby]

Allowing an option to avoid conflicts seems doable. Creating a globally unique name (within the graph) however is O(n). Would be good to know what level of guarantee we can to provide