Speed Up DecoderMaskedSelfAttentionTest (#19531)

### Description
The unit tests take 19 minutes to run (in debug build) because of too
many combinations. I reduce the combinations and remain good test
coverage. After the change, the test can finish in 51 seconds.

Before:
[----------] 2 tests from DecoderMaskedSelfAttentionTest
[ RUN      ] DecoderMaskedSelfAttentionTest.Test_fp32
[       OK ] DecoderMaskedSelfAttentionTest.Test_fp32 (394086 ms)
[ RUN      ] DecoderMaskedSelfAttentionTest.Test_fp16
[       OK ] DecoderMaskedSelfAttentionTest.Test_fp16 (747035 ms)
[----------] 2 tests from DecoderMaskedSelfAttentionTest (1141122 ms
total)

After:
[----------] 2 tests from DecoderMaskedSelfAttentionTest
[ RUN      ] DecoderMaskedSelfAttentionTest.Test_fp32
[       OK ] DecoderMaskedSelfAttentionTest.Test_fp32 (21057 ms)
[ RUN      ] DecoderMaskedSelfAttentionTest.Test_fp16
[       OK ] DecoderMaskedSelfAttentionTest.Test_fp16 (30653 ms)
[----------] 2 tests from DecoderMaskedSelfAttentionTest (51710 ms
total)


### Motivation and Context
Reduce test time, and improve build pipeline efficiency.