[Relay] Change Default "opt_level" of Sequential from 2 to 0

[Johnson9009]

Current default "opt_level" of Sequential is 2, this value will lead to bug when sequential nested.
The below test case can't pass, through the opt_level of pass SimplifyExpr is 0, but the opt_level of wrapper sequential is 2, so the pass SimplifyExpr will not be run.

def test_nested_sequential_with_scoping():
    def before():
        x = relay.var("x", shape=(1, 16, 16, 16), dtype="float32")
        w = relay.var("w", shape=(32, 16, 3, 3), dtype="float32")
        y = relay.nn.conv2d(x, w, padding=(1, 1))
        y = relay.reshape(y, newshape=(1, 16, -1))
        y = relay.reshape(y, newshape=(4, 8, -1, 16))
        y = relay.reverse_reshape(y, newshape=(32, 0, -1))
        return tvm.IRModule.from_expr(y)

    def expected():
        x = relay.var("x", shape=(1, 16, 16, 16), dtype="float32")
        w = relay.var("w", shape=(32, 16, 3, 3), dtype="float32")
        y = relay.nn.conv2d(x, w, padding=(1, 1))
        y = relay.reshape(y, newshape=(32, 16, 16))
        return tvm.IRModule.from_expr(y)

    z = before()
    passes = [
        tvm.transform.Sequential([relay.transform.SimplifyExpr()]),
    ]
    with tvm.transform.PassContext(opt_level=1):
        zz = tvm.transform.Sequential(passes)(z)

    expected = relay.transform.InferType()(expected())
    assert tvm.ir.structural_equal(zz, expected)

Actually from the comments of class Sequential, it seems we think the default value of this opt_level should be 0, but the code is not changed.

tvm/python/tvm/ir/transform.py

Lines 189 to 193 in 804ba27

	opt_level : Optional[int]
	The optimization level of this sequential pass.
	The opt_level of a default sequential pass is set to 0.
	Note that some of the passes within the Sequantial may still not be executed
	if their opt_level is higher than the provided opt_level.

@tqchen @zhiics

[comaniac]

The CI gets timeout. Not sure if the slowdown was due to opt_level=0 or just a CI issue. Maybe simply re-trigger CI and see how it goes.