linalg.fill shouldn't have a fixed result

[gabrielrodcanal]

Hi there,
Haven't really checked the custom format in the operation definition in MLIR, but I've found an example in which -> type($res) is optional (I'm referring to

xdsl/xdsl/dialects/linalg.py

Line 439 in ae2054d

"`outs` `(` $outputs `:` type($outputs) `)` `->` type($res) attr-dict"

). See the example here: https://github.com/Xilinx/mlir-air/blob/82d23361cb3843e5d8fc01d746d513f25a224125/test/airhost/51_air_mmult_2x2_channel_broadcast/air.mlir#L30