[TOPI][Bugfix] Make semantics of empty axis in squeeze consistent with Relay

[wzh99]

This PR fixes #11697. The analysis of the bug is elaborated in #12400. The key problem is whether axis in squeeze means doing nothing or squeezing all one-dimensional axes.

In #12400, I provide two possible fixes. I choose to fix TOPI finally. The meaning of axis is clearly described here (commit #2020):

tvm/include/tvm/relay/attrs/transform.h

Lines 319 to 327 in fb7cf97

	TVM_DECLARE_ATTRS(SqueezeAttrs, "relay.attrs.SqueezeAttrs") {
	TVM_ATTR_FIELD(axis)
	.describe(
	"The axis to squeeze in the input tensor."
	"If `axis = None`, all axis of dimension 1 get squeezed;"
	"Else, the dimension in axes get squeezed."
	"It is an error if an axis does not has dimension 1.")
	.set_default(NullValue<Array<Integer>>());
	}

The later commit to TOPI (#2147), however, does not follow this description. It seems that there is a mistake in this commit and should be corrected.

CC: @masahi

[masahi]

Can you add a test?

[wzh99]

@masahi Of course. It seems that I should add the test here:

tvm/tests/python/topi/python/test_topi_transform.py

Lines 943 to 948 in a9f7c32

	@tvm.testing.requires_gpu
	def test_squeeze():
	verify_squeeze((1, 2, 3, 4), 0)
	verify_squeeze((1, 2, 1, 4), None)
	verify_squeeze((1, 1, 1, 4), (1, 2))
	verify_squeeze((1, 1, 1, 1), None)

I have prepared the test case verify_squeeze((1, 1, 1, 1), ()) to see whether TOPI correctly handles empty axis. However, I have a problem with this testing function before pushing the commit: Why is it decorated with @tvm.testing.requires_gpu, instead of @tvm.testing.uses_gpu as the other testing functions in this file? In other words, can it run on a CPU-only target?

[masahi]

hmm it's weird that squeeze is only tested on gpu targets:

tvm/tests/python/topi/python/test_topi_transform.py

Line 954 in a9f7c32

for target in ["cuda", "opencl"]:

Can you add llvm there and replace requires_gpu with uses_gpu?

[wzh99]

@masahi I have pushed the commits, including the new test case and llvm target.