[Rewriter] Create a way to create an initializer in the rewrite pattern (and core onnxscript)

[justinchuby]

Allow this usage in the rewrite pattern

def replacement(x):
    tensor = ir.tensor(..., name="new_name")
    return op.Add(op.Initializer(tensor), x)

def replacement_when_y_is_const_node(x):
    # Q: How do I get that const node?
    ...

def replacement_transpose_initializer(x):
    tensor = x.const_value
    transposed = ir.tensor(tensor.numpy().T)
    return op.Initializer(transposed)

def Initializer(tensor: ir.TensorProtocol, name: str | None = None) -> ir.Value:
    # When name is None, the value name will be taken from the tensor
    # Otherwise the name is specified in name
    # Optionally we can also generate a name if nothing is specified, but I am not sure if that's error-prone
    ...

cc G. Ramalingam (@gramalingam) kunal-vaishnavi

[gramalingam]

Completed by #2019