Changed ff_dim to ff_dim_t, added in nonnegative_int type

[victorli2002]

Description of changes:

• draft
• renamed ff_dim to ff_dim_t
• added a nonnegative_int type with some unit tests
• added a relative_ff_dim_t type for now (unused)

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 91.52542% with 25 lines in your changes missing coverage. Please review.

Project coverage is 78.17%. Comparing base (1d5140d) to head (f9fe89b).

Files with missing lines	Patch %	Lines
...utils/src/utils/nonnegative_int/nonnegative_int.cc	87.06%	15 Missing ⚠️
lib/local-execution/src/legion_tensor_shape.cc	0.00%	4 Missing ⚠️
lib/kernels/src/array_shape.cc	0.00%	2 Missing ⚠️
lib/kernels/src/legion_dim.cc	0.00%	2 Missing ⚠️
lib/local-execution/src/ops/gather.cc	0.00%	1 Missing ⚠️
lib/local-execution/src/ops/layer_norm.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           repo-refactor    #1533    +/-   ##
===============================================
  Coverage          78.16%   78.17%            
===============================================
  Files                860      863     +3     
  Lines              27994    28473   +479     
  Branches             770      779     +9     
===============================================
+ Hits               21881    22258   +377     
- Misses              6113     6215   +102     

Flag	Coverage Δ
unittests	78.17% <91.52%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
lib/kernels/include/kernels/legion_dim.h	25.00% <ø> (-3.58%)	⬇️
lib/local-execution/src/ops/linear.cc	0.00% <ø> (ø)
...p-attrs/include/op-attrs/dim_ordered/dim_ordered.h	81.69% <ø> (-0.67%)	⬇️
lib/op-attrs/include/op-attrs/ff_dim_t.h	100.00% <ø> (ø)
lib/op-attrs/test/src/op-attrs/dim_ordered/zip.cc	100.00% <ø> (ø)
...ls/include/utils/nonnegative_int/nonnegative_int.h	100.00% <100.00%> (ø)
.../test/src/utils/nonnegative_int/nonnegative_int.cc	100.00% <100.00%> (ø)
lib/local-execution/src/ops/gather.cc	0.00% <0.00%> (ø)
lib/local-execution/src/ops/layer_norm.cc	0.00% <0.00%> (ø)
lib/kernels/src/array_shape.cc	20.83% <0.00%> (ø)
... and 3 more

... and 111 files with indirect coverage changes