Fix valgrind leaks (with minor refactor)

[alonre24]

• In DAG_OP init, we allocate inkeys and outkeys arrays. When we parse model run command, we allocate inkeys and outkeys arrays as well (what causes the leaks). This PR contains a temporary fix, this will be refactor soon.
• In DAG_commandParser, the following line causes a leak:
  RedisModuleString *mangled_key = RedisModule_CreateStringFromString(NULL, key);
  A call for RedisModule_FreeString was added.
• The allocation of DAG_device_ops array in RunInfo_Init is redundant (it is initialized in every shallow copy).
• The function Tensor_DataTypeStr allocates memory on the heap to store the string that represents the tensor type, and it wasn't freed in one place. This allocation is replaced with stack allocation for char array - thus we save allocations and get rid of the leaks .

[codecov]

Codecov Report

Merging #549 (ec613bf) into master (896799d) will increase coverage by 0.14%.
The diff coverage is 92.20%.

@@            Coverage Diff             @@
##           master     #549      +/-   ##
==========================================
+ Coverage   76.01%   76.16%   +0.14%     
==========================================
  Files          25       25              
  Lines        5508     5433      -75     
==========================================
- Hits         4187     4138      -49     
+ Misses       1321     1295      -26     

Impacted Files	Coverage Δ
src/DAG/dag.c	86.88% <ø> (-0.27%)	⬇️
src/backends/tensorflow.c	67.16% <0.00%> (-0.21%)	⬇️
src/err.c	97.56% <ø> (+2.21%)	⬆️
src/tensor.c	83.57% <85.71%> (+0.82%)	⬆️
src/command_parser.c	90.00% <87.50%> (-0.63%)	⬇️
src/DAG/dag_parser.c	87.64% <100.00%> (+0.14%)	⬆️
src/background_workers.c	87.30% <100.00%> (-0.44%)	⬇️
src/model.c	70.38% <100.00%> (+0.83%)	⬆️
src/run_info.c	73.14% <100.00%> (+3.19%)	⬆️
src/script.c	65.65% <100.00%> (+0.83%)	⬆️
... and 1 more

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[lantiga]

Hi @alonre24 , can you provide a small description of the changes and what the leak was due to?

[lantiga]

LGTM, good catches