[TE] Make elem_offset of the buffers created by te.extern a variable to avoid crash

[masahi]

Currently, when compiling MobileBERT on x86, we get a crash from

tvm/src/tir/transforms/arg_binder.cc

Lines 102 to 106 in 11d22bd

	if (is_zero(arg->elem_offset)) {
	ICHECK(is_zero(value->elem_offset))
	<< "Trying to bind a Buffer with offset into one without offset "
	<< " required elem_offset=" << arg->elem_offset
	<< ", provided elem_offset=" << value->elem_offset;

.

This is caused from the subgraph attached in the test. Since #11341, concatenate is implemented as a TE extern on x86. te.extern(...) doesn't explicitly set the elem_offset parameter of input / output buffers it creates, which ends up elem_offset of these buffers being the default value, 0. This causes a problem from the above check in BindBuffer, since the subgraph does create a non-zero elem_offset slice from a buffer whose elem_offset is set to zero.

Since we don't require buffers created in te.extern(...) to have zero elem_offset, we can avoid this problem by explicitly setting elem_offset to be a variable there. This fix was found by @Lunderberg and I'm sending this PR on behalf of him. He also suggested making the strides parameter an array of Var as well, but I didn't do it to keep the change minimal.

cc @Lunderberg @tkonolige @shtinsa

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[shingjan]

LGTM!

[Lunderberg]

LGTM, and thank you for making this change! For context on the fix, this didn't cause an issue previously, because the default elem_offset=0 could bind to an entire buffer. It only became necessary when the buffer being bound as an argument to the te.extern was a view into a larger backing allocation, which could occur after inlining.