[mlir][sparse] Use variable instead of inlining sparse encoding (#72561)

Example:

#CSR = #sparse_tensor.encoding<{
  map = (d0, d1) -> (d0 : dense, d1 : compressed),
}>

// CHECK: #[[$CSR.*]] = #sparse_tensor.encoding<{ map = (d0, d1) -> (d0
: dense, d1 : compressed) }>
// CHECK-LABEL: func private @sparse_csr(
// CHECK-SAME: tensor<?x?xf32, **#[[$CSR]]**>)
func.func private @sparse_csr(tensor<?x?xf32, #CSR>)