ISQ in parallel for Metal, too

[EricLBuehler]

Because #492 ensures that ISQ now happens on the CPU, we can try to run ISQ in parallel for Metal, too.

Tagging: @Luddo183, this refs #393.

[github-actions]

Code Metrics Report

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 Language            Files        Lines         Code     Comments       Blanks
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 Dockerfile              1           34           25            0            9
 Happy                   1          442          369            0           73
 JSON                    9           21           21            0            0
 Python                 32         1256         1075           37          144
 TOML                   16          444          403            1           40
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 Jupyter Notebooks       1            0            0            0            0
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 (Total)                            156          117           23           16
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 Markdown               19         1363            0         1021          342
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 |- Rust                 3          151          135            6           10
 (Total)                           1712          320         1027          365
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 Rust                  121        37354        33750          684         2920
 |- Markdown            59          668           13          617           38
 (Total)                          38022        33763         1301         2958
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 Total                 201        40914        35643         1743         3528
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