[Bug] [lang] Fix outer_product output matrix shape

[archibate]

Related issue = fixes #1460

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Thank @gmh14 for reporting!
I made the test to use different size in rhs and lhs to prevent regression.

[codecov]

Codecov Report

Merging #1461 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1461      +/-   ##
==========================================
- Coverage   66.69%   66.64%   -0.05%     
==========================================
  Files          38       38              
  Lines        5305     5310       +5     
  Branches      948      948              
==========================================
+ Hits         3538     3539       +1     
- Misses       1607     1611       +4     
  Partials      160      160              

Impacted Files	Coverage Δ
python/taichi/lang/matrix.py	91.19% <ø> (ø)
python/taichi/lang/ops.py	92.41% <0.00%> (-1.34%)	⬇️
python/taichi/core/util.py	21.72% <0.00%> (ø)

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[yuanming-hu]

LGTM! Thanks for fixing this!