specific seed is only respected after tf has been initialised?

greta should respect the RNG seed specified in R (greta explicitly passes the latest RNG state into TF, and increments the RNG state in R), and it does, but apparently only *after* TF has been initialised.

In the below reprex, the values af a, b, and c should all be the same, but the value for a (seed set before fir greta array created and initialisation of python) is different than the other two.

``` r
library(greta)
#>
#> Attaching package: 'greta'
#> The following objects are masked from 'package:stats':
#>
#> binomial, cov2cor, poisson
#> The following objects are masked from 'package:base':
#>
#> %*%, apply, backsolve, beta, chol2inv, colMeans, colSums, diag,
#> eigen, forwardsolve, gamma, identity, rowMeans, rowSums, sweep,
#> tapply
set.seed(1)
x <- normal(0, 1)
#> ℹ Initialising python and checking dependencies
#> ✓ Initialising python and checking dependencies ... done
#>
a <- calculate(x, nsim = 1)

set.seed(1)
y <- normal(0, 1)
b <- calculate(y, nsim = 1)

set.seed(1)
z <- normal(0, 1)
c <- calculate(z, nsim = 1)

a
#> $x
#> , , 1
#>
#> [,1]
#> [1,] 1.304048
b
#> $y
#> , , 1
#>
#> [,1]
#> [1,] 0.4455042
c
#> $z
#> , , 1
#>
#> [,1]
#> [1,] 0.4455042
```

^{Created on 2021-06-09 by the [reprex package](https://reprex.tidyverse.org) (v2.0.0)}

``` r
sessionInfo()
```
```
R version 3.6.2 (2019-12-12)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS Mojave 10.14.6

Matrix products: default
BLAS: /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRlapack.dylib

locale:
[1] en_AU.UTF-8/en_AU.UTF-8/en_AU.UTF-8/C/en_AU.UTF-8/en_AU.UTF-8

attached base packages:
[1] stats graphics grDevices utils datasets methods base

other attached packages:
[1] greta_0.3.1.9012

loaded via a namespace (and not attached):
[1] Rcpp_1.0.6 highr_0.8 compiler_3.6.2 pillar_1.4.3
[5] prettyunits_1.1.1 base64enc_0.1-3 tools_3.6.2 progress_1.2.2
[9] digest_0.6.27 evaluate_0.14 jsonlite_1.7.2 lifecycle_1.0.0
[13] tibble_3.0.3 lattice_0.20-38 pkgconfig_2.0.3 png_0.1-7
[17] rlang_0.4.11 reprex_2.0.0 Matrix_1.2-18 cli_2.5.0
[21] rstudioapi_0.13 yaml_2.2.1 parallel_3.6.2 xfun_0.22
[25] coda_0.19-4 knitr_1.33 withr_2.4.2 styler_1.1.1
[29] vctrs_0.3.8 fs_1.5.0 globals_0.14.0 hms_1.1.0
[33] grid_3.6.2 reticulate_1.20 glue_1.4.2 listenv_0.8.0
[37] R6_2.5.0 processx_3.5.2 parallelly_1.25.0 rmarkdown_2.8
[41] callr_3.7.0 clipr_0.7.1 purrr_0.3.4 whisker_0.4
[45] magrittr_2.0.1 ps_1.6.0 htmltools_0.4.0 backports_1.1.5
[49] codetools_0.2-16 tfruns_1.5.0 ellipsis_0.3.2 future_1.21.0
[53] tensorflow_2.4.0 crayon_1.4.1
```