fix dividing by zero and assertion, and add test-blas0 in make test on x86_64

[katsu560]

After gq merged to master, make test failed by exception by zero dividing and assertion.
It caused by cgraph->n_threads is zero in ggml.c ggml_graph_compute().
and assertion is GGML_ASSERT(!ggml_is_transposed(a)); in ggml_mul_mat().

And test-blas0 is tested on x86_64 system with using OpenBLAS, but make test is failed.
I fixed tests/CMakeLists.txt to do test-blas0 with command line options '0 0 0'.

test3 is too slow on my old 4-cored PC with running 8 threads setting.
So, n_threads parameter is changed by GGML_NTHREADS or command line option.
It changes test3 time from 50.10sec to 2.35sec.

before:

$ GGML_NLOOP=1 GGML_NTHREADS=4 make test
Running tests...
Test project /home/user/github/gpt/ggml/build
      Start  1: test-vec0
 1/11 Test  #1: test-vec0 ........................   Passed    4.11 sec
      Start  2: test-vec1
 2/11 Test  #2: test-vec1 ........................   Passed   12.14 sec
      Start  3: test-grad0
 3/11 Test  #3: test-grad0 .......................***Exception: Numerical  2.20 sec
      Start  4: test-mul-mat0
 4/11 Test  #4: test-mul-mat0 ....................Subprocess aborted***Exception:   2.09 sec
      Start  5: test-blas0
 5/11 Test  #5: test-blas0 .......................***Failed    2.15 sec
      Start  6: test-mul-mat2
 6/11 Test  #6: test-mul-mat2 ....................   Passed    4.68 sec
      Start  7: test0
 7/11 Test  #7: test0 ............................   Passed    2.17 sec
      Start  8: test1
 8/11 Test  #8: test1 ............................***Exception: Numerical  2.18 sec
      Start  9: test2
 9/11 Test  #9: test2 ............................   Passed    7.26 sec
      Start 10: test3
10/11 Test #10: test3 ............................Subprocess aborted***Exception:   2.30 sec
      Start 11: test-svd0
11/11 Test #11: test-svd0 ........................   Passed    2.11 sec

55% tests passed, 5 tests failed out of 11

Total Test time (real) =  43.44 sec

The following tests FAILED:
          3 - test-grad0 (NUMERICAL)
          4 - test-mul-mat0 (Subprocess aborted)
          5 - test-blas0 (Failed)
          8 - test1 (NUMERICAL)
         10 - test3 (Subprocess aborted)
Errors while running CTest
Output from these tests are in: /home/user/github/gpt/ggml/build/Testing/Temporary/LastTest.log
Use "--rerun-failed --output-on-failure" to re-run the failed cases verbosely.
Makefile:70: recipe for target 'test' failed
make: *** [test] Error 8

after:

$ GGML_NLOOP=1 GGML_NTHREADS=4 make test
Running tests...
Test project /home/user/github/gpt/ggml/build
      Start  1: test-vec0
 1/11 Test  #1: test-vec0 ........................   Passed    4.00 sec
      Start  2: test-vec1
 2/11 Test  #2: test-vec1 ........................   Passed   11.85 sec
      Start  3: test-grad0
 3/11 Test  #3: test-grad0 .......................   Passed    2.40 sec
      Start  4: test-mul-mat0
 4/11 Test  #4: test-mul-mat0 ....................   Passed    2.42 sec
      Start  5: test-blas0
 5/11 Test  #5: test-blas0 .......................   Passed    2.80 sec
      Start  6: test-mul-mat2
 6/11 Test  #6: test-mul-mat2 ....................   Passed    4.89 sec
      Start  7: test0
 7/11 Test  #7: test0 ............................   Passed    2.45 sec
      Start  8: test1
 8/11 Test  #8: test1 ............................   Passed    2.27 sec
      Start  9: test2
 9/11 Test  #9: test2 ............................   Passed    7.16 sec
      Start 10: test3
10/11 Test #10: test3 ............................   Passed    2.35 sec
      Start 11: test-svd0
11/11 Test #11: test-svd0 ........................   Passed    2.31 sec

100% tests passed, 0 tests failed out of 11

Total Test time (real) =  44.95 sec

please confirm this pull request.

[ggerganov]

Pushed an alternative fix 216b172