ggml : add check for grad_accs

[danbev]

This commit adds a check for grad_accs in ggml_graph_get_grad and ggml_graph_get_grad_acc functions. This is necessary to avoid segfaults when grad_accs is not initialized.

The motivation for this change is that I find it nice to be able to print out a computation graph using ggml_graph_print but this function segfaults when grad_accs is not initialized:

(gdb) p g1
$2 = (ggml_cgraph *) 0x7ffff66004b0
(gdb) p *g1
$3 = {size = 2048, n_nodes = 1, n_leafs = 2, nodes = 0x7ffff6600500,
grads = 0x0, grad_accs = 0x0, leafs = 0x7ffff6604500,
visited_hash_set = {size = 4099, used = 0x7ffff6610518,
keys = 0x7ffff6608500}, order = GGML_CGRAPH_EVAL_ORDER_LEFT_TO_RIGHT}
(gdb) p ggml_graph_print(g1)
=== GRAPH ===
n_nodes = 1

Program received signal SIGSEGV, Segmentation fault.
0x0000555555579775 in ggml_graph_get_grad (cgraph=0x7ffff66004b0,node=0x7ffff6600340)
    at /ggml/ggml/src/ggml.c:5990
5990  return igrad != GGML_HASHSET_FULL && ggml_bitset_get(cgraph->visited_hash_set.used, igrad) ? cgraph->grads[igrad] : NULL;